Someone can correct me if I'm wrong, but my understanding is that in GPC you're allowed to sink the bar into the chest on the descent and use your chest/ribcage for rebound when you get the press call, so long as you do not double dip (ie no further sinking/downward movement after you starting pressing). Very useful trick for raw bench.
Edit: to be fair I was watching some old 80s IPF lifting and it was allowed then too. Under the plain wording of the current IPF rules, I think the rule is the same as GPC and sinkage into the chest is also fine so long as it is not "after the bar is motionless in such a way as to make the lift easier." But I don't think it should be. In practice it might be interpreted differently. One of the PA refs here might be able to clear that up.