A33: It works like a readied action - so you could use it to save yourself from a crit just as if you had readied an action to move "if he attacks me."

I don't think "if he confirms a crit on me" is a valid trigger though (your character would have no way of knowing that until it happened without metagaming) so I would say you should burn the hero point before the GM rolls to confirm, but your GM should be weighing in on a corner case like that anyway.