Fencer: Here is a position in Hostage Chess I played recently; I control the Black pieces. In the gamescore, the Asterisk (*) indicated a piece is being dropped on the board. Two piece abbreviations in parenthesis indicate that a piece is being exchanged for another piece, and that shorthand is followed by the piece that is being dropped and the square on which that piece is being dropped. EX: 7.(N-B) B*c5 means that White exchanged a white bishop for a black knight, and the bishop is dropped on c5.

r3kb2/pp3pp1/1qpp2n1/6p1/2P1P1N1/bB1P1nP1/
PPPpN3/RKrR4 w - - 0 29

I invite you to set it up and play along.

In White's prison: black rook.
In Black's prison: white pawn, bishop, queen.

Free Pieces: white bishop.

White is in check. White to move.

I want to continue the process of executing the smothered mate.
29. Choices between RxR or NxR. The c1 square is being occupied and attacked 3 times, and defended 3 times, but the king is one of the defenders. The black Knight on f3 is threatening mate as soon as the black pawn on d2 vacates the square. When the pawn promotes, anyone in the prison (in this case the rook) can be redeployed. Because the rook is giving a contact check, no square is left for the freed bishop or queen to intervene. They, in this case, are useless. The decision to drop the piece could have been to drop the rook anywhere along the 1st rank. Also I could have kept the white queen in prison and sacrificed my queen to re-deploy it. But would that have been a good idea? My bishop is sitting en prise at a3, which a moment ago would have forced a mate for black if unanswered or would have weakened the b2 square and allowed the bishop to reinforce the c1 square for attack. I have to come up with a plan.

29a1. RxR--Rd1xc1, d2xc1 = *R (pawn goes to prison in exchange for Rook) + ; 30. Ne2xc1, Nf3-d2 #.
29a2. RxR---Rd1xc1, d2xc1 = R +; 30.Kb1xc1, Qb6-e3+ and either dropped piece is captured with a contact check by the queen at d2, and then the rooks can be exchanged with check and a won game. […(R-R) R*d1+, and queen & rook can force a drop, and the knight comes to d2 anyway for a smother.]
29b. NxR----Ne2xc1, d2xc1 = R +; 30. Kb1xc1, and the e3 square is protected by the N at g4. So re-deploy the pawn, 30…(P-P)*d2+; 31. Choice of RxP or moving the king to b1 again. There is a powerful sequence which brings the white king to a5, which I forgot to annotate, but here is the position, which entails NOT taking the rook on d2, but instead forcing checks and captures with Ba3xb2+; KxB, (N-B)B*c3+; Kxc3, Qd4+; Kb4, d2-d3 (discovered check with Bf8)+; Ka5 (forced), Qb3 + with at some point an exchange and drop of pawns at *b4 with check, and here is the FEN:

r3kb2/pp3pp1/1qp3n1/Kp1p2p1/2P1P1N1/1B1P1
nP1/P1PR4/R7 w - - 0 1

Hopefully that will give you an idea of how play moves in Hostage Chess.