Wikipedia
A choke-out is a hand-to-hand combat tactic involving the use of a chokehold to cause syncope, or temporary loss of consciousness, at which point the choke is released. Common chokeholds in grappling used to accomplish a choke-out are known as the rear naked choke, arm triangle, triangle choke, and the guillotine.
The mechanics behind choke-outs are disputed. It has been explained as resulting from directly constraining blood flow to the brain. A competing theory involves compression of the baroreceptors of the carotid artery, confusing the body into thinking blood pressure has risen. Due to the baroreflex, this causes vasodilation, or widening of the brain's blood vessels intended to relieve high pressure. Since no blood pressure increase has actually occurred, the dilation causes a dramatic decrease in blood pressure to the brain, or brain ischemia, which then causes loss of consciousness.
Choke-outs should not be confused with erotic asphyxiation or the fainting game, wherein a person loses consciousness intentionally in order to experience a particular sensation. A choke-out should also not be confused with medical conditions that cause fainting without the application of a chokehold.