Crazyhouse

The variant where nothing ever really leaves the board.

The rules

When you capture a piece, it changes colour and joins your reserve. On any turn,
instead of moving, you may drop a piece from your reserve onto any empty
square.

Restrictions on drops:

• pawns may not be dropped on the first or the eighth rank
• a promoted piece reverts to a pawn when captured — you capture a queen, you
  receive a pawn
• you may drop a piece to give check, and even to give checkmate

Everything else follows ordinary chess.

What changes

There are no quiet positions and no safe kings. A hole in the squares around
your king is not a long-term weakness; it is somewhere a knight lands next move.

Trading pieces does not simplify. It hands your opponent ammunition, and a
"winning" endgame does not exist because the board never empties.

What to watch for

Count the enemy reserve before every move. A defence that holds against
their pieces on the board may collapse against a knight they have been holding
for ten moves.

Guard squares, not pieces. Any square next to your king that no pawn covers
is a landing site. Knights and pawns are the most dangerous things to concede,
far more than rooks.

Clocks matter here more than anywhere else: the positions are sharp and the
tactics are endless, so many games are decided on time.