When a man sits on a woman's face with his ass ("male" facesitting), or the opposite ("female" facesitting), or a man sits on another man's face with his ass ("gay" facesitting) or a woman sits on another woman's face with her ass ("lesbian" facesitting).
Usually the person under the ass licks it (an act which is called "rimming" or "rimjob").
Sometimes the man which facesits a woman also slaps her pussy for his pleasure (an act which is called "cuntslapping" or "cuntbusting").

The greatest thing to happen to the human race. Along with FemDom facesitting is great sucking that vagina and FemDom facesitting is good too because having a slave forced to be sat upon is a great view. I also love scat facesitting where a mistress will poop into the human toilets mouth(slave) and they must eat it and I love it.