The idea of boxing out is to hold your ground, and not let the opponent around you to get a rebound. You aren't allowed to displace the opponent. Girls get called for this all the time, especially at the 3pt line. They shove their butt right into the shooters stomach and physically move them, and their coaches always fight about it screaming they did exactly as they are taught to do. Some coaches really do teach their players to displace the opponent and try to push them further away from the basket.