Oracle Text

Target creature can't block this turn.

Draw a card.

Card Rulings

10/4/2004 Must be used before blockers are declared in order to affect blocking decisions. You can’t wait to see what your opponent declares and then try to stop them.
10/4/2004 You can use it after combat or even on a creature which can’t possibly block this turn because it’s that player’s turn to attack, but it generally has no effect other than to let you draw a card.