You just chain-cast Shadowbolts.

Need loads of +crit, though.

EDIT: If you are PvPing, don't wear Frozen Shadoweave.

what bananaz said.
even after the recent patch fire won't do more damage unless you have lots more fire mages than warlocks and shadow priests in your raid. shadow does more damage though slightly less efficiently.
bananaz is also right that destro requires lots of crit which is something the FSW set doesn't have, nor does it have +hit. however, if you can get plenty of +hit and +crit from the rest of your gear the pure damage of the FSW set is really good.