You may play that card during your next turn without spending any Action Points. Discard it afterward. As for the card itself, the ability to gain a free Attack could be very useful. According to designer Mike Fitzgerald, the Attack card gained via Change of Momentum is intended to be a totally free play, i.e., no Nitro or AP cost and refers to the current attack only. If none of the damage from this card is stopped, the action is out of the ring until your turn ends and you gain 1 extra Action Point this turn.
Card is expensive.
The first point of damage that this card does that is not stopped comes randomly from the other player's hand instead of his or her deck. When that player has no cards left in his or her hand, the rest of the damage comes from his or her deck. As such, this card could be good more for the nuisance value than for the actual amount of damage it deals. Same ability, but stronger card than Front Face Lock or Wicked Hair-Pull. You may discard a Wrestler card from another player's corner to add that Wrestler's Reputation to this attack's damage number.