When you capture a piece, you may place a ring on your capturing piece. That ring lets the piece move like the captured piece one time in the future.
A ring must add a move your piece does not already have. A queen can take a knight ring, and can take a pawn ring for pawn-only powers like en passant.
A piece can wear only one ring. If you earn a new ring, it replaces the old one. You may also decline a ring.
If you capture a piece that already has a ring, you may choose either the captured piece’s type or the ring it was wearing, as long as that ring adds a new ability.
When a piece uses its ring move, the ring is removed. Ring moves can give check, and a pawn using a ring move to reach the back rank still promotes.