When one is not able to defecate for a set period of time thus resulting in strong urges to masturbate which, in turn, causes a euphoric sensation and an explosion of the bowels (anal leakage is also common during the process of constabation).
4 people in a hot tub:
Judy: Wow this is relaxing.
Patty: Yeah it sure is.
Joe: Mike! What the hell are you doing?
Mike: Oh man sorry guys. I haven't shit in five days so I'm practicing the fine art of constabation. To tell you the truth I am very aroused at the moment to the point where I feel as though I might possibly be able to drop a douce in the near future.