When you take a breath, you fill your lungs with oxygen. The length that you can hold your breath is how long you can go off the oxygen in your lungs. It will be slowly turned to CO2 by your cellular metabolism processes. Normally, you clean out the waste by breathing out and get new oxygen to use by breathing in.

During that entire time, your heart is still pumping blood around and the oxygen is still being used. Your brain uses a massive amount of oxygen. When you choke someone out, you're stopping the oxygen in their blood from getting to the brain.