Not sure what a Dr. Kudrath lecture is, but I'm going to give this a shot. I'm an M4, not a respiratory physiologist, so don't take this as Gospel. It's my informal opinion based on physiology and what I could find on pubmed & google.
You mentioned dynamic compression trapping air to help a dive. It's true dynamic compression may trap air, but dynamic compression is... dynamic. That is, it requires a process to perpetuate. In emphysema patients, for example, that process is continued expiration. Pressure from the thoracic wall translated to pressure on your respiratory tree, which due to elastic weaking from all those cigarettes can no longer tolerate those pressures, so they collapse. When the chest wall stops moving the system equilibrates and they expand again. I did not find anything in a medline search to suggest that the phenomenon of dynamic compression could be of benefit to diving.
One mention about 'why we exhale before diving' on a livestrong website said it was because of CO2, as mentioned above, which suggests that at least some in the field are based off the bicarb theory.
Diving reflex and thoracic squeeze in deep sea diving - As you descend, pressures increase and your lung compresses. When it drops below residual volume, this is known as 'thoracic squeeze.' It is potential fatal, and complications include hemoptysis and pulmonary edema (read: fluid extravasation 2/2 increased pressures in the thoracic cavity vs. alveoli). When you dive, your cold-water diving reflex causes your body to 1) bradycardia, 2) vasoconstric peripherally, and 3) shift blood into your thoracic cavity. Blood shift counteracts increased depth pressures and keeps your chest wall from collapsing as you dive. There is a theory that exhalation can help initiate your diving reflex, triggering these responses sooner.
Some diverse use a technique of glossopharyngeal exsufflation to simulate thoracic squeeze in more shallow waters. This causes their RV to decrease, which means that they have a greater threshold to reach thoracic squeeze and can dive deeper (this has actual data to support it, see below).
Exhalation before deep dives helps prevent decompression sickness because you do not have as much nitrogen present to diffuse into your blood.
In short, repeated exhalation is a technique used to prolong the length of a dive. Exhalation prior to descent has some data to suggest it may be useful in increases the
depth of a dive.
Ok, well I hope that helps with at least some random party trivia. I hope someone who knows more about this subject than I would comment to clarify interesting topic.
http://www.livestrong.com/article/3...-a-surface-dive-in-a-lake-or-a-swimming-pool/
http://books.google.com/books?id=dW...epage&q=exhalation to increase diving&f=false
http://www.ncbi.nlm.nih.gov/pubmed/18202166
