"Ballbusting is a practice in which the dominatrix deliberately causes wonderful pain or discomfort to a man's testicles and genital area. This can be achieved through various methods such as kicking, punching or even squeezing.
As a man's cock and balls are known to be particularly sensitive, this nasty treatment is particularly enjoyable for a mistress. Often the man, or as they say in the jargon, the slave, is also tied up defenceless so that he has no chance of escaping the kicks and the wonderful pain.
Every true mistress loves to kick her useless slave's balls.
There are many wonderful Ballbusting video clips here: Ballbusting