An Expert 10×10 Puzzle Solved with Forced Chain in 39 Steps

This 10×10 board rates as expert (difficulty score 1849). Solving it from scratch takes 39 logical steps, using 7 techniques: Single cell, Row/column exclusion, Adjacency clear, Region confinement, Line confinement, Shape squeeze, Forced chain. Every step below is forced: nothing here is a guess.

1. First, every remaining candidate in this region touches B6, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
2. Next, every remaining candidate in this region touches C5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, every remaining candidate in this region touches B9, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
4. After that, marking A1 would force D2, G2, H2, and 2 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then D7, E7, F7, and 4 more cleared (every open cell left in this region sits in row 7, so its marker has to land there), then B8, B10, C8, and 2 more cleared (together, two regions have open cells only in columns B and C), and then region 9 would be left with no open cell for its marker. So A1 can't be the marker there.
5. Now, if B1 were the marker, it would force C6 (the last open cell in region 7), then D2, G2, H2, and 2 more cleared (every open cell left in this region sits in row 2, so its marker has to land there), then A8, A9 and A10 cleared (every open cell left in this region sits in column A, so its marker has to land there), and region 9 would end up with no open cell for its marker, which can't happen. So it can't go there.
6. From there, that one can't be the marker: at C1 it would force B5 (the last open cell in region 7), then A7 (the last open cell in region 6), and region 9 would be left with no open cell for its marker.
7. Following that, that one can't be the marker: at A2 it would force D7, E7, F7, and 4 more cleared (every open cell left in this region sits in row 7, so its marker has to land there), then B8, B10, C8, and 2 more cleared (together, two regions have open cells only in columns B and C), and region 9 would be left with no open cell for its marker.
8. First, marking B2 would force C6 (the last open cell in region 7), then A8, A9 and A10 cleared (every open cell left in this region sits in column A, so its marker has to land there), and then region 9 would be left with no open cell for its marker. So B2 can't be the marker there.
9. Next, if C2 were the marker, it would force B5 (the last open cell in region 7), then A7 (the last open cell in region 6), and region 9 would end up with no open cell for its marker, which can't happen. So it can't go there.
10. Then, marking E2 would immediately leave row 1 with no open cell for its marker, so it can't be the marker there.
11. After that, if A3 were the marker, it would force D7, E7, F7, and 4 more cleared (every open cell left in this region sits in row 7, so its marker has to land there), then F2 cleared (every open cell in row 1 belongs to the same region, so that region's marker has to be somewhere in this row), then B8, B10, C8, and 2 more cleared (together, two regions have open cells only in columns B and C), and region 9 would end up with no open cell for its marker, which can't happen. So it can't go there.
12. Now, that one can't be the marker: at B3 it would force C6 (the last open cell in region 7), then A8, A9 and A10 cleared (every open cell left in this region sits in column A, so its marker has to land there), and region 9 would be left with no open cell for its marker.
13. From there, that one can't be the marker: at D2 it would force G4, H4, I4, and 1 more cleared (every open cell left in this region sits in row 4, so its marker has to land there), then G5, H5, G6, and 3 more cleared (every open cell in row 3 belongs to the same region, so that region's marker has to be somewhere in this row), then B5 cleared (every remaining candidate in this region touches B5), and the chain continues, and region 9 would be left with no open cell for its marker.
14. Following that, marking C3 would force B5 (the last open cell in region 7), then A7 (the last open cell in region 6), and then region 9 would be left with no open cell for its marker. So C3 can't be the marker there.
15. First, if E1 were the marker, it would force F4, G4, H4, and 2 more cleared (every open cell left in this region sits in row 4, so its marker has to land there), then F3, G3, H3, and 8 more cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then D3 (the last open cell in row 3), and the chain continues, and region 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
16. Next, that one can't be the marker: at F1 it would force E4, G4, H4, and 2 more cleared (every open cell left in this region sits in row 4, so its marker has to land there), then G3, H3, I3, and 7 more cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then D5, E5, D6, and 2 more cleared (every open cell in row 3 belongs to the same region, so that region's marker has to be somewhere in this row), and the chain continues, and region 9 would be left with no open cell for its marker.
17. Then, marking G1 would force E4, F4, H4, and 2 more cleared (every open cell left in this region sits in row 4, so its marker has to land there), then F3, H3, I3, and 5 more cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then D5, E5, F5, and 4 more cleared (every open cell in row 3 belongs to the same region, so that region's marker has to be somewhere in this row), and the chain continues, and then region 9 would be left with no open cell for its marker. So G1 can't be the marker there.
18. After that, if H1 were the marker, it would force J2 (the last open cell in row 2), then E4, F4 and I4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), then D5, E5, F5, and 4 more cleared (every open cell in row 3 belongs to the same region, so that region's marker has to be somewhere in this row), and the chain continues, and region 9 would end up with no open cell for its marker, which can't happen. So it can't go there.
19. Now, that one can't be the marker: at I1 it would force G2 (the last open cell in row 2), then E4, F4 and H4 cleared (every open cell left in this region sits in row 4, so its marker has to land there), then D5, E5, F5, and 4 more cleared (every open cell in row 3 belongs to the same region, so that region's marker has to be somewhere in this row), and the chain continues, and region 9 would be left with no open cell for its marker.
20. From there, marking J1 would force E4, F4, G4, and 2 more cleared (every open cell left in this region sits in row 4, so its marker has to land there), then F3, G3, H3, and 7 more cleared (every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row), then D5, E5, F5, and 4 more cleared (every open cell in row 3 belongs to the same region, so that region's marker has to be somewhere in this row), and the chain continues, and then region 9 would be left with no open cell for its marker. So J1 can't be the marker there.
21. Following that, place a marker at F2. Region has exactly one open cell left. Placing here clears the rest of row 2, column F, and its region. Placing here also clears its 2 touching neighbours.
22. First, place a marker at D1. Row 1 has exactly one open cell left. Placing here clears the rest of row 1, column D, and its region.
23. Next, every open cell left in this region sits in column E, so its marker has to land there; that clears every other open cell in the column.
24. Then, every open cell in row 3 belongs to the same region, so that region's marker has to be somewhere in this row; that clears every other open cell in the region.
25. After that, every remaining candidate in this region touches I4, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
26. Now, if H4 were the marker, it would force J3 (the last open cell in region 4), then I10 (the last open cell in column I), then G7 (the last open cell in column G), and the chain continues, and row 8 would end up with no open cell for its marker, which can't happen. So it can't go there.
27. From there, place a marker at E4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column E, and its region.
28. Following that, marking B5 would force J6 (the last open cell in row 6), then G7 cleared (every open cell left in this region sits in row 7, so its marker has to land there), then C9 (the last open cell in region 8), and the chain continues, and then row 8 would be left with no open cell for its marker. So B5 can't be the marker there.
29. First, place a marker at C6. Region has exactly one open cell left. Placing here clears the rest of row 6, column C, and its region. Placing here also clears its 1 touching neighbour.
30. Next, place a marker at G7. Region has exactly one open cell left. Placing here clears the rest of row 7, column G, and its region. Placing here also clears its 1 touching neighbour.
31. Then, place a marker at A5. Region has exactly one open cell left. Placing here clears the rest of row 5, column A, and its region.
32. After that, every remaining candidate in this region touches I9, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
33. Now, that one can't be the marker: at H3 it would force J9 (the last open cell in row 9), and column I would be left with no open cell for its marker.
34. From there, if J3 were the marker, it would force H9 (the last open cell in row 9), and column I would end up with no open cell for its marker, which can't happen. So it can't go there.
35. Following that, place a marker at I3. Region has exactly one open cell left. Placing here clears the rest of row 3, column I, and its region.
36. First, that one can't be the marker: place it at J8, and row 9 would be left with no open cell for its marker.
37. Next, place a marker at H9. Region has exactly one open cell left. Placing here clears the rest of row 9, column H, and its region.
38. Then, place a marker at J10. Region has exactly one open cell left. Placing here clears the rest of row 10, column J, and its region.
39. Finally, place a marker at B8. Region has exactly one open cell left.

The key move here was Forced chain. Once you can spot that, this board falls quickly. ∎