The reserve waits for the line to break: the first time your opponent captures one of your pieces, a knight rides in on the empty back-rank square nearest your king's file.
You draft Reserve Cavalry in Buff mode. Passive: it works on its own the whole time you hold it, with nothing to activate.
As a pieces card, it summons, revives, promotes, or converts a piece.
“Held back for exactly this moment.”