Text (UL+errata): At the beginning of your upkeep, if you have no cards in hand, sacrifice ~this~. If you do, target opponent discards his or her hand. [Oracle 2001/03/07]
It counts the number of cards in your hand during the resolution of the triggered ability. [D'Angelo 1999/05/01]
If this card is not in play at the time it is to be sacrificed, then you can't and the opponent does not discard their hand. [WotC Rules Team 1999/03/18]
