How to solve this 6×6 logic puzzle

6×6 · sharp · 18 steps

This 6×6 board rates as sharp (difficulty score 1144). Solving it from scratch takes 24 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.

First, every remaining candidate in this region touches row 2, column 5 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Next, every remaining candidate in this region touches row 2, column 2 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Then, every remaining candidate in this region touches row 5, column 3 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, marking row 1, column 1 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Now, marking row 1, column 2 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
From there, marking row 1, column 3 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
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 — clearing every other open cell in the region.
First, every open cell in row 2 belongs to the same region, so that region's marker has to be somewhere in this row — clearing every other open cell in the region.
Next, every remaining candidate in this region touches row 3, column 2 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
Then, every remaining candidate in this region touches row 5, column 2 — so whichever candidate ends up marked will clear it, and it can be ruled out now regardless of which one wins.
After that, marking row 1, column 5 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
Now, marking row 1, column 6 forces a chain of moves that breaks another row, column, or region, so it can't be the marker there.
From there, place a marker at row 1, column 4. Region has exactly one open cell left. Placing here clears the rest of row 1, column 4, and its region. Placing here also clears its 1 touching neighbour.
Following that, place a marker at row 2, column 1. Region has exactly one open cell left. Placing here clears the rest of row 2, column 1, and its region.
First, place a marker at row 4, column 2. Region has exactly one open cell left. Placing here clears the rest of row 4, column 2, and its region.
Next, place a marker at row 3, column 5. Region has exactly one open cell left. Placing here clears the rest of row 3, column 5, and its region.
Then, place a marker at row 6, column 3. Region has exactly one open cell left. Placing here clears the rest of row 6, column 3, and its region.
Finally, place a marker at row 5, column 6. Region has exactly one open cell left.

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