Astral Projection aside for a moment, generally speaking, it seems obvious to me the answer is... Oxygen. I first started to think about this when I noticed through repeat observation (of both myself and others) that during waking there is often a deep intake of breath. The more suddenly the waking, the sharper the breath. This got me thinking on bodily processes.

To be physically alive, two of the main processes needed are respiration and circulation. Thinking about emergency paramedics will make that clear, as the first lifesaving techniques are resuscitation and palpitation. Other bodily systems are needed too of course, but in this sense they are secondary. You need oxygen to your cells! Why? Because it acts as the 'link' between your physical and etheric and astral bodies, allowing your consciousness to 'anchor' physically, and the energy of your soul to come through. (Conversely, you need to exhale to get carbon dioxide out of your cells as it is a 'blockage' to this.)

Perhaps oxygen is more like a 'dimensional gateway' for energy in that sense. It is after all a pure gaseous element, having a high physical vibration (unlike carbon dioxide which is 'weighed down' by carbon.) And it makes sense of why yogic exercises concentrate so much on breath to influence energy flow. For example, concentrating on 'breathing in' to a particular area will energize it, and breathing out conversely.

After all, when you don't get enough oxygen, you 'pass out' (of your physical body!) You get 'pins-and-needles' or numb sensations when you cut off blood supply to a limb (you don't think those pins and needles are the sensation of your energy body trying to re-anchor or something? Hmmm...) Those who have had Near Death Experiences are drawn back when their heart starts pumping the good old O2 through their bodies...

I'm sure there are numerous other examples, but I hope this point is clear enough for anyone to see the truth in it. I've heard it hinted at by various sources, but strangely enough haven't heard anyone actually spelling it out. It is certainly more than implied in many Eastern practices, but far less commonly in mentioned in Western philosophies.

So what about during Astral Projection? Obviously we are still breathing when projecting, but the shallow and slower breathing make a big difference in enabling the majority of our consciousness to leave the physical body and 'go off and do other things'. In sleep our physical bodies use a minimal energy requirement to keep us alive. To that extent only a small portion of our consciousness remains with the body, so it only needs a little oxygen.

I wanted to mention the oxygen aspect to this because I think it can explain a few uncertainties to do with projection phenomena. For example, fear and excitement mobilize the biological mechanisms, often increasing the heartbeat and pumping more blood into your veins and cells. Projectors frequently report that becoming afraid or over-excited, especially toward the start of any experience, can 'shock' one back into the physical body or otherwise end the experience.

I suggest the reason this happens is due to the phenomena of 'repercussion', where what is felt in one body can be felt in the other. This usually has a stronger effect with close proximity to the physical body. So excitement or fear increase your heart rate and thus blood flow and oxygen levels, 'drawing' you back. It's also worth mentioning that using breathing effectively can be very helpful in obtaining the relaxation required for the projection process.