I'll take a crack at writing one, since it is my promo.
When you buy this, you gain an Action card costing up to $4 from the Supply and set it aside face up. If you did set it aside, then at the start of your next turn, play that Action card. This doesn't use up your default Action for the turn. In order to remember to play the card on your next turn, you may want to turn it sideways or diagonally, turning it right side up when you play it. If you move the Action card after you gain it but before you set it aside (e.g. by putting it on top of your deck with Watchtower, from Dominion: Prosperity), then Summon will "lose track" of it and be unable to set it aside; in that case you will not play it at the start of your next turn. If you use Summon to gain a Nomad Camp (from Dominion: Hinterlands), Summon will know to find the Nomad Camp on your deck, so you will set it aside in that case (unless you have moved it elsewhere via another ability).
