That is called a Brachial Stun maneuver. There are a lot of nerves on the sides of the neck. A smack there is like hitting your funny bone, except it shorts out the bodies complete nervous system for a split second. Think of a natural tazer.
The thing is, if done wrong and too hard, it causes death due to breaking the neck.
A 90lb woman can drop a 300lb man with that very easily.