I had always assumed that constriction was where a snake squeezes and each breath exhaled left less air in the lungs as the snake continued to tighten. Upon further reading however, I have seen that there is evidence that constriction is actually cutting off the blood flow to the prey's brain or puts tremendous stress on the circulatory system.
I would love if anyone had some additional information on this, I have found a few articles, but nothing conclusive. Thanks!
Nat Geo