Text (JU+errata): Flashback - Sacrifice a creature. ; Name a nonland card. Target player reveals his or her hand and discards all cards with that name. [Oracle 2005/08/01]
You name the card during resolution, then your opponent reveals their hand and discards if appropriate. There is no way for your opponent to do anything between you naming the card and them discarding. [Judgment FAQ 2002/05/28]