Set aside up to 4 cards from your hand face down (on this). The next time you gain a Victory card, trash this, and put the set aside cards into your hand at end of turn.
Cage is a Treasure-Duration card from Plunder. It is a one-shot that lets you thin your deck by temporarily pseudo-trashing up to four cards. The cards return to your deck when you start greening, however, so you may need to be prepared for a larger-than-usual decrease in deck quality at that stage of the game.
The cards go to your hand after drawing your regular hand of 5 cards for next turn.
For example you might set aside two Estates and two Coppers on a Cage on an early turn; then on a late turn, buy a Province, trash the Cage, and add the two Estates and two Coppers to your hand at end of turn.
Other rules clarifications
If you set aside nothing with this, it will still stay in play until you gain a Victory card.
If gaining a Victory card causes you to play this (e.g. you gain a Province, then gain a Cage with Haggler, and play it with Mining Road), that won't trigger the Cage (it triggers off the next Victory card you gain).
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Set aside up to 4 cards from your hand face down (on this). The next time you gain a Victory card, trash this, and put the set aside cards into your hand at end of turn.