This spell allows the caster to duplicate the damage suffered by a single creature within spell range on the previous round. Damage is precisely duplicated in type and amount—in other words, the same damage rolls, saving throws, ability checks, and so on occur in the same way as in the previous round. The damage may have been caused by any source, not just magical attacks.
For example, an evil fighter is struck by two arrows, taking 3 and 4 points of damage, parries a thrust by a ranger with a sword of sharpness, is backstabbed by a thief with weakly poisoned dagger (missing the saving throw vs. poison and taking 15 points of damage), and takes more 11 points of damage after making his saving throw against a finger of death spell, all in the same round that a wizard casts afterclap against him. The next round, the fighter takes another 33 points of damage from the afterclap (3+4+15+11 = 33), in addition to whatever damage his enemies inflict.
The material component of an afterclap is a bit of stiff rubber or gum that is pulled until it snaps back.
Notes: Very rare in the Forgotten Realms setting and for chronomancers; virtually unknown elsewhere.