Crazyhouse (wild 23) is like two-player Bughouse. When you capture
a piece, it becomes a piece of your color that you can place later.
On your turn, you can either make an ordinary chess move, or place
one of the pieces in your holding (e.g. "N@g5"). You can't put
pawns on the first or eighth rank. Promoted pawns revert to pawns
upon being captured. You can drop in check or checkmate. Crazyhouse
has its own rating.
Versions of BlitzIn prior to 2.5 don't handle examining Crazyhouse
games correctly. BlitzIn 1.72 and earlier won't display your holdings
at the edge of the board.