So far, there is no real scientific evidence that supports any breathe-up technique as being "safe". There are conflicting studies. During my last conversation with Dr. Neal Pollock at DAN, breathe-up techniques are all forms of hyperventilation to some extent. The classic practice of deep rapid breathing will result in a spiked heart rate prior to the dive. This spiked heart rate might be more problematic than other breathe-ups in which the diver is able to maintain a more relaxed state with a lower heart rate.
There is a lot of anecdotal and unsupported evidence regarding freediving techniques. What we do know, is that as time passes, the body will use oxygen. Eventually, enough oxygen will be used so that the diver will lose consciousness. This will happen whether the diver reduces the CO2 and doesn't feel the urge to breathe or if the diver is able to hold back the chest contractions that you mention. Yes, what you describe sounds like chest contractions that result from the body attempting to breathe.
Official United States Navy studies of breath-hold diving resulted in a recommendation of 1 minute being the "safe zone" of apnea. After 1 minute, every freediver is rolling the dice and taking a gamble. My quickest blackout was 2:38 when doing 5:30 breath-holds was my norm.
Psychology and relaxation play the biggest parts in freediving. I've had students do very well with one technique that worked well for them, then after researching or taking other freediving courses, they became psyched out by the philosophy they researched or practiced. The fact that the freediving community is in love with their deep divers and tries to imitate them, like golfers imitating the swing of their champions, would be fine, if not for the fact that the group with the worst safety record is the group that most freedivers are looking to for training advice. What I mean by that is that when you have competitions with more than 10 shallow-water blackouts and a cardiac arrest, I'm not sure that is where a diver will find the safest training advice?
In the past, most divers were both freedivers and scuba divers who found an enjoyment in diving in whatever discipline they were doing on a given day. Staying safe and not pushing the limits resulted in many divers never having a blackout throughout an entire lifetime spent in the water. Today, there is a push for depth, time and distance much of which is garnered by a high level of fitness and adaptation toward bettering the abilities of the freediving mammals we naturally are, but in the end is based upon a lot of luck and divine care.
Growing up, we just dove. Today's freediver is often more obsessed with technology and numbers than they technical diving community in which I have more of an interest at the present time. Part of my loss for the appeal of freediving was all of the "competition", the formation of freediving agencies, the prima donna-like petty rivalries and jealousies that have muddied the community for a couple of decades now like a stubborn silt-out. Added to that, the scuba community no longer sees freediving as being basic diver training. Freediving is becoming moved into a position of mystique in which only a few gurus know how rather than an activity that every diver is introduced to during training. Worse, many of the instructors introducing students to snorkeling and freediving in open water courses, can't even do those activities proficiently because they were never taught. Freediving is more mysterious than ever before, when it should be more widely practiced and understood.
Regarding more of your questions, the phrase, "Slow is smooth, smooth is fast," should be how you approach freediving. If you feel the urge to breathe on ascent, relax. Don't swim fast toward the surface. Cruise up slowly. You don't want to demand more oxygen or spike your heart rate with fast swimming that will demand more oxygen to feed the muscles. Stay relaxed during all phases of diving. You want to be loose, lazy, fluid, calm, tranquil, at peace in mind and body, and graceful.
I've done a lot of solo freediving between 150 and 200 feet as well as solo apnea training when I was young and I'm lucky to have reached age 41. I started snorkeling as a kid and doing serious freediving in my late teens and early 20's into my 30's. Freediving without a buddy or a spotter is a high risk activity. It's like skydiving without a reserve parachute. However, in an age in which we've become safety-obsessed and even elitist about how ridiculously safe we are being in daily life and diving, if you do dive alone practice common sense. Limit your time underwater. Carry redundant cutting tools. Listen to your instincts. Conserve energy and make sure you stop diving when tired and save energy for the return swim.
Personally, I limit myself to 2 minutes and 100 feet when alone. Others would disagree with my parameters or disagree with solo freediving altogether, but I believe that individuals have the right to decide what risks they are willing to take. I don't ride motorcycles - bloody death machines. If you are new to freediving, I'd suggest taking a course or diving with an experienced freediver to develop safe habits and improve your technique.
Hope this helps.
