Usually, units can only be commanded to fire at visible enemies. Force-Firing (Sometimes called Force-Attacking) makes a unit fire to a certain spot where no enemy is present, or is present but not visible.
The default force-firing button is CTRL. It should be held while a unit is selected, and then the player should click somewhere on the battlefield. When the force-fire button is held, the cursor turns red (or blue for mine layers).
Force-firing can also be used to kill one's own units or a teammate's units, but this doesn't make sense and is not strategic.
Some units where force-firing is particularly relevant are: