An enema is the procedure of introducing fluids into the rectum using a hose or appropriate enema syringe.
In BDSM play enemas have three main roles, and are enjoyed by a range or Masters or Mistresses for differing reasons.
The enema is used to clean out the rectum and lower area of the colon by liquefying the body's waste and allowing the slave to expel it easily when the go to the toilet. This is often done to make anal sex easier, since there is less obstruction in the rectum making entry easier and more pleasant for both slave and Master.
Because the anus and rectum is a very sensitive area, the enema can provide stimulation that is completely different from other sorts of play. The initial push of the fluid followed by the pressure gives a unique feeling, which can often be exciting for the slave.
The private nature of the anus, the unusual vulnerability that the slave feels, and the slave's exposure while receiving an enema allows the slave to feel unique humiliation. Large enemas or forcing the slave to hold the enemas for more than a short period can also be used as a form of torture or punishment.
Enema is very popular in Japanese SM play.