The document discusses the "Shrinking Net theory" which explains how the size of the net a goalie needs to protect changes based on their position in the crease. When a goalie is at the top of the crease, the dimensions of the net are 3 feet high by 5 feet wide. As the goalie moves away from the goal line, the size of the net they need to cover decreases. Playing in the butterfly position allows goalies to maximize their coverage of this shrinking net area.