Let's talk about this concept without the use of any diving equipment whatsoever. Imagine that your body mass is neutral without a wetsuit, without weights, without scuba in ten feet of water. Imagine being totally relaxed, so much so, that you are either unconscious or dead. What position do you think you'd be found in? Upright? Upside down? On your front? On your back?
Being so relaxed in scuba gear, chances are you would turn turtle if you lapsed into unconsciousness or died wearing heavy tanks because of the relationship of weight to your center of gravity. The photo of the diver who was presumably murdered by her husband in Australia wearing a single AL tank shows her sinking on her back. Her death was captured by accident when another diver shot a photo of his buddy. You can see her in the background on her back.
If recovery divers decided they wanted to make your body perfectly neutral with a 0° angle of attack, they could do so by adding lift bags at strategic points to accomplish this. You are so relaxed, being dead, that you can offer no help by positioning your limbs or moving your center of gravity. Once they've made you perfectly neutral, if a limb is even slightly moved out of that position, the center of gravity will change and they would have to reposition a lift bag to compensate.
Now, let's say you are not dead or unconscious and you aren't wearing any gear and you are neutral at 10 feet. Get vertical in the water. How do you do that? Your brain quickly sorts out the force of gravity and your body's relationship to the spacial environment. You know which was is down, which way is up and your brain sends messages to your muscles to apply varying levels of tension to balance yourself in a vertical position. Get perfectly straight, like a soldier standing at attention. That will be more difficult and will require a greater degree of spacial processing and muscle tension to fine tune that position. If we want you to achieve a perfect position, it may require coaching, practice, muscle memory formation and heightened awareness. Now, try to hold that position prone, on your back and upside down. Your body will have to change how the muscles contract, tense and balance you in each of those positions based upon your center of gravity. There is a reason that Olympic platform divers are in shape. Being able to change the body's position requires muscle tension and work. Gravity is acting upon the body and the body must work against gravity to achieve various positions.
Underwater, we still must contend with gravity. Thanks to our ability to be buoyed up by a force that is equal to the force that is pulling us down, we can achieve neutral buoyancy, but we still must work to achieve a certain position because we still have a center of gravity rather than uniform gravity.
Now, all the factors mentioned in the articles and in previous posts come into play to achieve a prone position in diving with your equipment and your individual body composition to achieve balance.
In the last GUE class I filmed, one student spent most of his time on the first two dives on his back on a platform at Dutch Springs. At one point he gave up and started trying to blow bubble rings. His GUE instructor was happy to join him and show him the correct technique for making bubble rings.
We got the student to employ the tricks of achieving trim and by the end of the Fundies class, he could maintain a proper prone position in good trim without turning turtle of crashing onto the platform. To do this, he needed to apply muscle tension and work to maintain that position. As the minutes ticked away on the dives, myelin began to guide his muscles to remember to keep his chest and knees flat, keep the head back against the manifold, keep the fins flat, etc.
The reason that you can carry a scuba tank on land and not fall over is because your muscles work to stabilize you and achieve balance as your center of gravity changes with each step or repositioning of how you carry your tank. The same happens underwater. Your stabilizer muscles come into play to orient you and you can make small corrections by muscle movement or tension.
During that Fundies class, I played an unconscious diver for the instructor's demonstration. To fake being unconscious, I went from perfect horizontal trim to sinking to land on my head and right shoulder. I didn't swim or move any limbs to do this, I just relaxed exhaled and let gravity take over. If I had wanted to stop that descent and go back into trim, all I would have had to do was put my head back against the manifold, tightened up my muscles, keeping the chest, knees and fins, flat, maintained a slight arch in the back and squeezed my glutes to stabilize my legs. When I landed, I was in a nearly perfect position, but upside down. Also, as I breathed, I was slightly rising off the platform during inhalation and gently tapping it as I exhaled. I dumped gas from my wing at that point to be more classically unconscious and let my body fall onto the platform. But, to go from perfectly trim and prone to head down only required a slight degree of muscle relaxation.
When freediving, I only have to adjust for 2 lbs. of ballast, but it still requires my body to work to achieve a needle position for descent or to change to a prone swimming position.
When I fall of my board when surfing, I must process where down is and where up is to protect my head and my muscles work to avoid or minimize impact with the bottom.
Even if you could get a scuba rig to sit in the water in a perfect diving position by itself, you would still have to do work to keep yourself positioned. If a rig sat that way, there would be less work for you, but every single scuba tank or set of doubles that I remember tended to roll upside down with the manifold or valve facing down when BCD's/wings weren't fully inflated or with the tanks down and BCD's/wings up when buoyancy systems were fully inflated. Most people want to float legs down. When you combine them, they help balance one another out. Whether a person wants to go head down or feet down, remain prone or turtle is simply adjusted with a few tricks of muscle work and muscle memory.
