I got to wallow in 5 of the 7. I had a second-birth instead of the re-birth (I know that because I was not a breach baby), and I never made it to cellular consciousness. My mythic experience was having my guts eaten by a large bird. I know what you're thinking, "That's sooo cool, I'm not going to try it."
But I did learn some things about what inhaling large amounts of oxygen does to my body. Breathing is an anesthetic. If you have an injury, particularly on the torso, your breathing will change. It will go to the place of injury and cover up the pain. Sometimes after an injury has healed, the breathing pattern which functioned as an anesthetic will still remain. One of the ways to diagnose chronic pain is by closely observing someones breathing patterns. Chronic pain often becomes numbness. A person with a chronic pain breathing pattern might not realize they are in pain until they start relaxing.
But weird hippy stuff and medical conditions aside, there is an important lesson here about the circulation of qi into the limbs. If you run long distance, or run up a large number of stairs (or anything which makes your breathing labored) the qi gates at your hips and shoulders will partially close (as will the perineum on the pelvic floor). This is because the heart is demanding more oxygen. This is why after a run, many people feel a lot of tingling in their arms and/or their legs--At the end of the run they experience their hip and shoulder gates reopening.
Likewise, simply breathing deeply will cut off qi circulation to the limbs. I'm guessing mammals developed this ability in order to increase our chances of survival in the event that we are loosing blood from a limb. It may even be that just before mammals "play dead" they have to get really scared and take a few really deep breathes.
In Taijiquan, Baguazhang, and Xingyiquan, we want optimal circulation of qi all over the body while we are moving at high speed. Without that optimal circulation throughout the body we will not develop the quality of being physically quiet and wild at the same time. All of these styles teach people to breath into the lower dantian and into the mingmen (lower back). At the beginning it is important that the breath not be constrained to the chest. Achieving this more relaxed style of breathing is largely a process of finding the "right" or "appropriate" dynamic postures.
However, it is possible to get "good" at breathing and not get good at circulation. This is because too much qi gets concentrated in the dantian and causes the hip gates to close. The hip gates are a bit like pressure valves which close automatically when the pressure gets too high. So the lesson is that breathing must be calm and gentle, perhaps even shallow, for optimal qi circulation to go into the legs. If qi isn't going into your legs, no matter how much training you do, you will not achieve quiet and wild at the same time, nor will you will achieve empty and full at the same time. Yes folks, even qi should not be horded.
The Daodejing says:
To be preserved whole, Bend.
Upright, then Twisted.
To be Full, Hollow Out.
What is worn out will be repaired.
Those who have little, have much to be gained; having much you will only be perplexed!