An Expert 6×6 Puzzle Solved with Forced Chain in 19 Steps

This 6×6 board rates as expert (difficulty score 1164). Solving it from scratch takes 19 logical steps, using 6 techniques: Single cell, Row/column exclusion, Adjacency clear, 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 B5, 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 E5, so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
3. Then, marking A1 would force B4 (the last open cell in region 4), then C6 (the last open cell in region 5), and then region 6 would be left with no open cell for its marker. So A1 can't be the marker there.
4. After that, if B1 were the marker, it would force C3 cleared (every open cell left in this region sits in column C, so its marker has to land there), then F3 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 E4, F4 and F5 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 row 4 would end up with no open cell for its marker, which can't happen. So it can't go there.
5. Now, that one can't be the marker: at C1 it would force B6 (the last open cell in region 5), and column A would be left with no open cell for its marker.
6. From there, marking D1 would force F2 (the last open cell in row 2), then E6 (the last open cell in region 6), then C3 (the last open cell in region 3), and then row 4 would be left with no open cell for its marker. So D1 can't be the marker there.
7. Following that, every open cell in row 1 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.
8. First, every open cell in row 2 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.
9. Next, 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.
10. Then, every remaining candidate in this region touches D2 and D4, so whichever candidate ends up marked will clear them, and they can be ruled out now regardless of which one wins.
11. After that, if E1 were the marker, it would force F6 (the last open cell in column F), then D3 (the last open cell in column D), then C5 (the last open cell in region 5), and row 4 would end up with no open cell for its marker, which can't happen. So it can't go there.
12. Now, place a marker at F1. Region has exactly one open cell left. Placing here clears the rest of row 1, column F, and its region.
13. From there, that one can't be the marker: at A2 it would force B4 (the last open cell in region 4), then C6 (the last open cell in region 5), and row 5 would be left with no open cell for its marker.
14. Following that, every open cell in column A belongs to the same region, so that region's marker has to be somewhere in this column; that clears every other open cell in the region.
15. First, place a marker at C4. Row 4 has exactly one open cell left. Placing here clears the rest of row 4, column C, and its region. Placing here also clears its 2 touching neighbours.
16. Next, place a marker at B2. Region has exactly one open cell left.
17. Then, place a marker at E3. Region has exactly one open cell left. Placing here clears the rest of row 3, column E, and its region.
18. After that, place a marker at D6. Region has exactly one open cell left. Placing here clears the rest of row 6, column D, and its region.
19. Finally, place a marker at A5. Region has exactly one open cell left.

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