Sparring is when two players co-operate with each other to create a smooth, realistic looking replay. Sparring requires players to have good balance, complete knowledge of joints and also requires communication with the other player.
While sparring, punches and kicks shouldn't be too hard that they put the other player off balance, and they should look as smooth as possible.
Most sparring matches take place on the default mod, while some take place in an environment created to specifically go with sparring and rarely in a parkour course.
Sparring can also be done in single player mode, but can be more time consuming, as the player must control both himself and Uke.