When Eric Garner repeatedly stated, I cant breathe, he was only able to do so because air was, in fact, moving through his trachea into his lungs. His breathing capacity may have been diminished but he was able to breathe at least up until he stopped talking. He died from the heart attack that he was having. One of the hallmark signs of a heart attack is difficulty breathing.
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But as all restraint training teaches, although the restrained person can speak, the airflow can be so diminished that the person is still suffocating, which leads to arrhythmias and death.
I had the banned choke hold demonstrated to me personally once. I tolerated it for about 2-3 seconds before I tapped out. I was able to squeak out a few noises but speaking was impossible because there was no air moving through my windpipe.