That's how they taught us to choke people during basic training in the USMC, and collectively referred to the chokes as "blood chokes" because you cut off blood supply to the brain. It's suprisingly easy to choke someone unconcious just by twisting the sides of their shirt collar, or grabbing someone from behind and squeezing the arteries on their neck with your forearm and bicep. The scariest part about doing it during training is that you don't know that you're about to blackout because you can still breath fine, it's just that none of the oxygen is getting to your brain.

Indeed. I've never lasted more than ~12 seconds, but was being conservative in my response as I don't have any facts on how long it takes.