A professional wrestling maneuver used to reverse offense by getting the person behind the user in front of him and on his back.
A snap mare may be applied when you have someone standing behind you, his belly at your back. You reach over your shoulder and apply a half nelson or a head lock to your opponent and press their chin to your shoulder. Then you kneel quickly and snap your opponent over your shoulder so your opponent flips over you and lands on his back in front of you open to an immediate continued attack.
It's not a particularly devistating move in wrestling circles - it's mostly used as a simple reversal - but it can seriously injure or kill someone when it is performed by untrained amateurs. See broken neck.
Plus it hurts to land on a hard surface instead of a wrestling mat