When Clay takes a round directly to the chest plate his breathing is paralyzed. This is actually a phenomenon that happens in rare cases, not just a "made for tv" thing. When body armor stops the bullet but the force that is still delievered shocks the system of wearer. A common treatment is to perform a sternum rub like what is shown the episode. Which helps restore normal function to the lungs.
The Bible verse that Ray speaks near the end of the episode is actually a combination of Romans 13:4 and Joshua 1:9
CASEVAC is Casualty Evacuation.
"Say again your last" is a way of asking someone to repeat themselves. Either because of broken radio signal or for clarification on what was said.