Hostage chess

Hostage chess is a chess variant invented by John Leslie in 1997. pieces are not eliminated from the game but can reenter active play through drops, similar to shogi. Unlike shogi, the piece a player may drop is one of his own pieces previously captured by the opponent. In exchange, the player returns a previously captured enemy piece which the opponent may drop on a future turn. This is the characteristic feature of the game.
Hostage chess has tactical subtlety and "tends to favour the attacker". In 1999, David Pritchard called the game "the variant of the decade". It was published in magazines "Nost-algia" (issue 375), "Eteroscacco" (86–88), and "Variant Chess" (32 and later). It was the "Recognized Variant of the Month" in January 2005 at "The Chess Variant Pages". 
The inventor, John Leslie, is also a philosopher. He mentions in his book "Infinite Minds" that the prevalence of chess variant inventions such as hostage chess has led to speculation that there could be infinite possible variations of chess. He contends these will necessarily exceed the capacity of the human mind.
Game rules.
Hostage chess follows all the standard rules of chess excepting how captured men are treated. Each player owns reserved spaces off the chessboard: a "prison" to the player's right, and an "airfield" to the player's left. There should be a clear boundary between each player's prison and the other player's airfield. Captured men are not removed from the game but are held in the capturer's prison. Instead of making a normal move, a player can perform a "hostage exchange" to "rescue" a man held prisoner by the opponent and "drop" the freed man back into play on the board onto an open square. The man exchanged for the dropped man is transferred from the player's prison to the opponent's airfield. On any turn, instead of making a normal move, a player can drop a man from his airfield into active play on the board. 
Hostage exchanges.
A hostage exchange is performed by transferring a man from one's prison to the opponent's airfield, then selecting and releasing a man from the opponent's prison and immediately dropping it onto an empty square on the board. The drop completes the turn. The man transferred must be of equal or greater value than the man released from prison and dropped. The relative piece values are: Q > R > B = N > P. (So, any man can be exchanged to free a pawn; whereas only a queen can be exchanged to free a queen.) A hostage exchange cannot be refused by the opponent.
Drops.
A drop can occur as part of a hostage exchange, or directly from a player's airfield. The square dropped to must be unoccupied. Additional drop rules: 
Pawn promotions.
A pawn can promote only to a Q, R, B, or N that is available in the opponent's prison. The promoting player selects which piece to release and promote to; the pawn is transferred to the opponent's prison. If the promoted piece is subsequently captured, it retains the type that it had when entering prison. 
So, if a pawn is on its player's 7th rank with no available piece to promote to:
Advantages over Chessgi.
According to David Pritchard: 
Notation.
Standard notation is used with some extensions:
Example game.
White: Frank Parr Black: David Pritchard 
1. d4 d5 2. c4
2... dxc4
3. Nc3 e5 4. d5 c6 5. e4 b5
6. dxc6
6... (P-P)*d4
7. *d7
7... Nxc6 8. Qxd4
8... Bxd7 
9. Qd1
9... (P-P)*d4
10. Nd5 Bb4+ 11. Bd2 Bxd2+ 12. Qxd2 Be6 13. a4
13... Bxd5 14. exd5
14... (B-B)B*b4
15. *c3 dxc3 16. bxc3
16... Bxc3 17. Qxc3
17... (N-B)B*b4
18. Qxb4 Nxb4
19. Rb1
19... (P-P)*d2+ ("diagram") 20. Kxd2
20... Qxd5+ 21. Kc1 Na2+ 22. Kc2 (P-P)*b3+ 23. Kb2 (Q-B)B*c3+ 24. Ka3
24... b4 
References.
Bibliography