The first click into the dangerous region can't harm you. But the following clicks should be
To choose a square use the button-lines that surround the region.
The left and bottom line clicks
determine the horizontal (bottom) and vertical (left) position of safety-marks.
To remove a
safety-mark repeat this procedure for the same coordinates.
If you dare to "open" a square, risking to be blown up, you should use the top (horizontal
coord.) and right (vertical coord.) button lines.