Given a vector like 1,1,1 and direction 1,0,0, will return 0,1,1. This is useful for velocity collision type events, where you want to cancel out velocity based on a normal. For this to work properly, direction should be a normal, but you can scale how much you want to subtract by scaling the direction. Ie, passing in a direction with a length of 0.5 will remove half the direction.