Boost

The Boost keyword represents a sequential effect with a specified duration. Boost N means “Once during your turn, you may pay N to put the top card of your deck facedown under this card.” A player can choose to resolve this effect any time they could use an activated ability (see 4.4, “Use an Activated Ability”).

No player can look at the front of a card that has been put facedown under another card at any time, not even their own.

If a card is under another card, it’s not considered to be in play. Putting a card under another card with Boost doesn’t count as playing that card.