Effectively, the combatant, hiding behind cover, makes a sighting of the enemy, meaning that a glance is made around a corner, from behind a tree or by raising one's head above a wall or similar defense. Then, in the following round, with a loaded weapon, the combatant moves, fires at the sighted location of the target and then returns to cover at once. The weapon may be loaded in the same round that the sighting occurs.
If the target has not changed hexes since the sighting, the missile or hurled weapon has a -1 chance of hitting when fired or thrown in this manner. If the target has moved, however, the attack will miss, unless a natural 2 is rolled (in which case, the target might be hit by accident so long as the target is within a 30-degree arc of its original position). This is meant to be a quirk, based upon the friendly fire rule found under Critical Hits & Fumbles.
The combatant firing in this manner is considered to be at -4 AC for the enemy's attack immediately after the sighted fire is attempted (remembering that D&D combat is a turn-based resolution of simultaneous attacks).
See Heightened Senses