Momentum is one of the three in play zones in UFS. Players start with an empty momentum. During the End Step, players may add attacks they played and that dealt damage, to their momentum. Cards are added to a player's momentum face-down unless otherwise noted; players can't look at their momentum but may change the order of the cards in their momentum as they choose.
Momentum is used as a ressource in UFS; many abilities, such as Powerful or Multiple require the player to discard a certain amount of momentum, meaning a certain number of cards from their momentum zone.
There are several ways to add cards to a player's momentum outside attacking; many actions and foundations have abilities that add cards to a player's momentum. Air, Fire and especially Chaos are particularly proficient at generating vast amount of momentum.