Captured pieces can be dropped back on the board instead of moving a piece.
All the rules and conventions of standard chess apply, with the addition of drops, as explained below.
A captured piece reverses color and goes to the capturing player's pocket. At any time, instead of making a move with a piece on the board, a player can drop a piece from their pocket onto an empty square on the board.
For example, a check that would result in checkmate in standard chess can be answered in Crazyhouse, if the defender can play a legal drop that blocks the check.
Drops resulting in immediate checkmate are permitted.
Pawns may not be dropped on the players' 1st or 8th ranks.
Promoted but captured pawns are dropped as pawns.
White to play, with two pieces in their pocket: a knight and a bishop. White can check the black king by dropping a bishop on c3.
Now black can block the check by dropping a knight on e5.
Final position, white to move.
An extension to the standard chess notation is used to record drops. Drops are notated by the piece type, where the piece type is blank for pawns, followed by an "@" symbol, then the destination square. For example, @d5 means "pawn is dropped on d5 from reserve".
Pawns and knights increase in relative importance in Crazyhouse, while rooks, queens, and bishops decrease in relative importance. If a king is put in check by any of the latter three pieces, from two or more squares away, dropping a pawn next to the king becomes defensively useful. A knight, on the other hand, cannot be blocked by anything and its offensive value is more manifest. That piece can be used effectively to maintain a strategic influence over a region.
After an early exchange of queens, it is usually unwise to reintroduce the queen too soon, particularly if she can be harassed by dropped minor pieces. Careful preparation is needed in order to reintroduce the queen to maximum effect
Pawns could be dropped deep in the enemy position where, for example, they can fork pieces or give an uncomfortable check.