To get back to the original question, about blowing out or not, you don't need to on ascent to prevent overexpansion, UNLESS you have taken a breath from a scuba unit at depth. But if you are ascending with your original breath, you need not blow any bubbles.
That being said, I have found that if I am in dire need for a breath while breath-hold diving, and feel that urge to breath as I am ascending, blowing out a small stream of bubbles delays that urge to breath.
There is a delicate balance for breath-hold diving between maximizing your dive time and getting into a shallow-water blackout situation. There are a couple of "tricks" free divers should be aware of, and that blowing bubbles is one of them. Another is to take off, and hold in your hand, your weight belt if you have over-extended your time and are in trouble. That way, if you go unconscious, you will automatically drop your weights and head to the surface if you are wearing a buoyant wet suit.
One other "trick" is to gradually get used to the breath-hold diving by diving on a cycle. I start by going down for about 30 seconds, and spend about a minute on the surface, then dive again. I don't hyperventilate--taking a maximum of three deep breaths before diving. After about five minutes on this schedule, I gradually increase the dive time, and find that I can stay down 1.5 minutes and more pretty easily after about fifteen minutes of diving this cycle. My body gradually adjusts to the breath-holding. This has prevented me from getting into a shallow-water blackout situation for a number of years now, and I thought I'd pass it along.
Shallow-water blackout can occur to anyone, often without warning. There are two mechanisms, one for deeper dives and one for extended horizonta breath-hold swimming. For deeper dives, as the pressure lessens and the oxygen levels decrease in the blood, there can be a reverse diffusion of oxygen out of the bloodstream and back into the lungs, triggering the blackout just under the surface (~10 feet) on ascent. For horizontal shallow swims, it is more complicated, but involves extreme hyperventilation, which blows off the CO2 from the blood stream. It is CO2 which triggers the "must breath" signal in our brain. Without the CO2, the oxygen level can get too low to maintain consciousness with no warning. The last thought that the diver/underwater swimmer has will be carried out by the body. If that last thought is to continue swimming, the body will even though the diver/underwater swimmer is unconscious. It has led to deaths, as the person continues to swim until the body cannot function, at which time there is only seconds to get the person out without brain damage. I know this as it occurred to me in my teen years as a swim team member, when I was in an underwater swimming contest. My friend, Tom had swam four lengths of a 20 yard pool, and I wanted to better that. So I told myself coming to that last turn that I would make the turn, push off, take one stroke underwater, surface and swim to the side of the pool. I had pre-programmed my brain to do this. And that is exactly what I did. Only, I don't remember anything after the pushoff on the turn until I was holding on to the side of the pool. Tom's mother, who was our swim team coach, stopped any further underwater swimming contests after that incident.
SeaRat
