A non-aggressive sideways move is a sideways move which does not kill any enemy checkers. White sideways 2nd black move. You must make a forward, diagonally forward, or aggressive sideways move. An aggressive sideways move kills enemy checkers. Once you have made a forward, diagonally forward, or aggressive sideways move, you must remove your marker from the board.
Forward move clears marker.
There is no limit to the total number of non-aggressive sideways moves you can make. You just can't make more than two non-aggressive sideways moves consecutively. This rule was instated because players have a tendency to approach each other and then stall the game by making an endless series of non-aggressive sideways moves. The marker forces players to engage each other in combat, no matter how reluctant they may be. Squaring was covered in the introduction, but here I give a few more details.