As I understand it, your carotid sinus controls the blood pressure in your brain. If you press or smack it hard enough, it thinks your BP is like 4 million over 80, so it shuts it down. Wiki expands a bit:
Carotid sinus reflex death is a potential etiology[5] of sudden death in which manual stimulation of the carotid sinus allegedly causes strong glossopharyngeal nerve (Vagus nerve is for aortic arch baroreceptors) impulses leading to terminal cardiac arrest. Carotid sinus reflex death has been pointed out as a possible cause of death in cases of strangulation, hanging and autoerotic strangulation, but such deductions remain controversial. Studies[citation needed] have also suggested that the carotid sinus reflex can be a contributing factor in other mechanisms of death by reducing blood pressure and heart rate, especially in the elderly or in people suffering from carotid sinus hypersensitivity. A carotid massage can also possibly dislodge a thrombus, or some plaque. This could lead to any number of life-threatening effects, including stroke.[6]
A smack won't kill you, but as far as I know most police departments aren't allowed to use rear naked chokes because doing it several times in a row has resulted in deaths.