Instead of making a normal move, a player may remove an empty square from the edge of the board. After a square is removed, more squares could be considered to belong to the edge of the board.
E.g., after h5 is removed, the square g5 is considered to belong to the edge of the board and could be removed in a turn, but g4 is not considered to belong to the edge of the board.
Rooks, bishops, and queens may not go across removed squares, knights can jump across removed squares just as they jump across squares with a piece on it.
An example: a few moves in, with squares removed by both players.
The following table shows which sqaures can be removed, and the square which they are attached to (which allows them to be removed)
|Can remove:||Because of:|
|B4||A4 or B5|