"Here, briefly, are the rules for Hostage Chess (courtesy of Variant Chess number 32).
You need a chess set and four beer mats (two for each player: one on your left - an airfield; and one on your right - a prison). Normal set-up and rules, except:
1) Once you capture a man you stick him in prison. He's a hostage.

2) Before each move, you may offer a hostage held by you in exchange for one held by the enemy. The hostage offered must be at least equal in value to the man you ask for (normal values of pieces, with B=N). The opponent can't refuse the swap.

3) The man you have claimed must immediately be placed on any vacant square (except pawns can't be put on 1st or 8th rank). This ends your turn.

4) The hostage you released is placed on the enemy airfield. It may be 'dropped' by the opponent on any vacant square at any time in the game, instead of a move.

5) A pawn can't be promoted unless there is a piece (N,B,R,Q) for which it can be exchanged. And if there isn't a piece available, it follows that a p on the seventh is not attacking (or giving check) to any piece diagonally ahead of it. Where a promotion is possible, the pawn changes places with the hostage of the player's choice. The released hostage becomes the promoted piece, and the pawn is removed to the enemy prison.

6) A pawn dropped on the second rank regains its two move option; and a rook dropped on a home corner square is deemed to be unmoved for castling purposes.

This fascinating game was invented by Professor John Leslie of Guelph, Canada."