While a choke does cut off/limit oxygen supply to the brain by limiting blood flow, which is air-flow as some have said, something else is usually going on.

The jugular veins are easier to occlude than the deeper arteries are, especially the internal carotid pair and the vertebral artery in the posterior neck. So, a choke actually TRAPS blood in the head, preventing it from flowing out at the same rate it's being pumped in.

This creates a spike in intra-cranial blood pressure, and THAT causes the KO even aster than hypoxia would. This can take, 3-4 seconds.

If it takes closer to 12-15 seconds, that's probably more from hypoxia.