Press this before committing to a risky test. When using a lifeline, testing a cell containing a mine does not end the game. When a lifeline is active, moving the mouse pointer over a cell will make it flash green (instead of the usual white). Use this option wisely, because you only have a limited number of them, and they severely affect your score and your time.
More options are found in the drop-down menu. Activate this menu by pressing the menu icon in the toolbar.
Pause Game [F3]
Stops the clock, hides the minefield.
New Game [Return], [F5], [Ctrl+N]
Stops the clock, hides the minefield, and shows the 'New Game' box.
One Mark in Cells
Some players have asked for the option to show just one mark in the cells, as it helps them count the mines.