Similar to a spotter in weight-lifting, a sputter is an individual whose job it is to monitor someone performing auto-erotic asphyxiation so that the act does not proceed to death.
Dude 1: So there I was in line at the grocery store when I had to fart.
Dude 2: Ya?
Dude 1: Well, my fart was more than a fart...I got some sputter butter...my underware was so messed-up.