Keys "s"=save "l"=load "b"=back once
Very good variant by John Leslie.
You may replace your captured pieces after first returning one of your own prisoners to the airfield from which your opponent may replace it.
The hostage you return must be of equal or greater value than the prisoner your replace.
Swap promoted pawns with pieces that your enemy has in prison. If you cannot, a pawn may not be taken to your far rank.
I found this works better if a paratrooper is not allowed to save you from checkmate.
Hostage Chess -- book and software
There are simple illustrations rather than strong opponents.
Bug reports? -- Thank you! Keep them coming!