Well, this is only a theory, i don't have brawl yet.
If Pit uses his up+b, and then he get hit, he CAN'T use his up+b again, so my idea is:
After Pit gets knocked out of the stage, when he is trying to recovery, go near to the edge of the stage and then use the chain, use the chain to stop his jumps, he will have no jumps left, and then, he will try to up+b, and the chain will hit he again...so he can't use the up+b again, and then he dies. If he tries to jump above you to avoid the chain, simply jump and use a fair/uair/nair.
Sorry for my bad english
If Pit uses his up+b, and then he get hit, he CAN'T use his up+b again, so my idea is:
After Pit gets knocked out of the stage, when he is trying to recovery, go near to the edge of the stage and then use the chain, use the chain to stop his jumps, he will have no jumps left, and then, he will try to up+b, and the chain will hit he again...so he can't use the up+b again, and then he dies. If he tries to jump above you to avoid the chain, simply jump and use a fair/uair/nair.
Sorry for my bad english