If you whack the carotid artery it will constrict to attempt to prevent damage to the brain from a surge in the local blood pressure. This can have several effects:

1. Temporary unconsciousness due to a sudden lack of oxygen to the brain.
2. Death or brain damage resulting from the artery constricting for too long - the older you are the more likely this is to happen.
3. Death or brain damage from vessels in your brain going pop because the artery failed to restrict blood flow quickly enough.

You know, I'd hate to have been one of the guys the ancient masters used to work out all these martial arts moves on. They must have ended up in really bad shape.

Needless to say, don't try this at home. (Go to someone else's home and raise their insurance costs.)