Hollow is a capricious game of heightened strategy for two players, red and blue. The objective of Hollow is to have more points at the end. You receive one point for each piece that remains on the board and something called the button exchanges hands throughout the game and awards the player who has it at the end of the game half a point. The game ends when no more moves of any kind remain.
Definitions: Hollow: A hollow is any set of cells partitioned by stones of either or both colors and possibly board edges.
Button: The button is a piece off the board worth half a point that the last player to create a hollow always receives or retains.
On a turn a player may either place a stone of his color, such that it does not alter the number of hollows or he may create a new hollow by removing the maximal group of stones, regardless of who they belong to, and leaving just the stones outlining the new region, often in conjunction with board edges.
Special thanks to Luis Bolaños Mures for creating a much simpler way of preventing hollows from being filled in with some classic efficient technical writing. My original plan was that players would always play in the uniquely largest region and that placements could never create hollows, but he instead suggested the current wording that seems so obvious now. My original approach would have led to a much less interesting game.
—description from the designer