Roller derby is a sport where women block and hit each other as they are roller skating around the rink. The purpose is to move around the track and to cause blocks to the opposing team. To oppose the team, it does not necessarily have to be a physical hit where bodies are hitting other bodies. One way is to skate where you would get in their way.
This would cause them to be stopped and they would not be able to go in the direction they wanted. Those who are blocking must be responsible for making the block legal. If a block is accidentally hurting the opponent, it would be the initiator of the block who would get into trouble.