Click a piece in your own airfield, then an empty square, to drop it. Click a piece in your own prison to offer a hostage exchange for one of equal or lesser value from the opponent's prison, dropping it immediately (the piece you gave up becomes the opponent's airfield piece).
Captured pieces never leave the game — they become "hostages" held by whoever captured them. On your turn, instead of a normal move you can ① drop a piece from your own airfield onto an empty square, or ② make a hostage exchange: give up one of your own hostages (an opponent-colored piece you captured) to the opponent's airfield, in return for rescuing one of your own color from the opponent's prison (of equal or lesser value) and dropping it immediately. Value order: Queen > Rook > Bishop = Knight > Pawn.
Simplifications: pawns cannot be dropped on the 1st or 8th rank. A pawn reaching the last rank auto-promotes to a queen (the real rule trades it for a prisoner — simplified here). The bishop's same-color-square drop restriction is omitted. Capturing the king directly ends the game instantly; check and checkmate are not enforced.