1) Load a puzzle.
2) Turn on pencilmarks (m).
3) While holding the CTRL button, click on the 1st square, 2nd square, ...
4) With the CTRL button still down, right click on the last square. Nothing happens.
5) Release the CTRL button.
6) Right click on the last square. The Place/Enable/Disable panel shows up.
7) Click on any digit to disable. The cross-hatching shows up instead.