The object of the game is to either live somewhere as White or to control the entire board as Black.
At the start of the game, players take turns placing black stones on the board. Instead of placing a black stone, a player may choose to play the game as Black, hoping that there are enough stones on the board to kill any white invasion.
Once the players' colors have been chosen, White plays first and tries to live somewhere. If he succeeds, he wins the game; otherwise Black wins.
To account for differing playing strength, there is an optional handicap system:
If a player has an n-stone handicap, and he chooses to play as black, he gets to place n additional black stones on the board before w plays his first move.
Notice that this handicap is useful even if the opponent chooses black. This is because the opponent had to choose black approximately n turns earlier than he would like.
This is a variant of the Shape Game (also called Skyboxing) that MXHero invented to fix the perceived inequality of the starting position in that game. Starting August 2010, it was tested extensively on KGS. The approximate number of black stones to play before switching is 17.
For information about the Shape Game, go to Shape Game Solid
A similar variant is listed on the Shape game page under 'another variant' (dating to 2004).