So how does having more oxygen in the mix you breath decrease deco time? Is it because by having less inert gases in the mix as you ascend you reduce the uptake of gas on your way up and therefore don't add even more N2 to your body?
What a great question! Now you are in the area of gas physics theory. I hope you enjoyed college physics and gas chemistry.
When you are at a depth of 20 fsw, and you are breathing 100 % O2, you are taking up no inert gas. Therefore this is the perfect depth at which to decompress the various inert gases from your blood, fat, muscles, and organs, because this is generally the deepest depth at which you can safely breathe pure O2. But you need to get there first.
You cannot quickly get to this depth from your MOD however. Normally your first deco stop must be around 1/2 of your MOD, arrived at from your MOD at 60 ft per min, and followed by 10 ft stops thereafter, normally of 1 min each for the first series of deep stops, followed by 2 min stops, then 3 min stops, then 4 min stops, etc. as determined by your favorite deco software.
To answer your question, then, it is by increasing the O2 fraction of the deco gas, when it is safe to do so, which tends to reduce the deco time at every given stop. The reason this works seems to correspond with the kinetic theory of gases, and the partial pressure theory, which seems to observe that gases in a mix act independently of each other.
Decompression of a given gas in the mix, resulting from the differential or pressure gradient between the partial pressure of a given gas dissolved in your blood compared with the partial pressure at which you are breathing it, takes place in your lungs across the avioli. It is an observation of the dynamics of decompression based on experimentation. It can be explained by theories of gas solubility and gas permeability across membranes (your avioli) having different partial pressures on either side of the membrane.
Higher concentrations of gases appear to migrate from higher pressure to lower pressure. This is why you introduce a higher fraction of oxygen, in order to induce a lower partial pressure of inert gas(es). This appears to speed up the decompression process. It is analogous to mopping up water with a sponge in your kitchen.
You should never lose track of what is happening in terms of dynamics of molecular particles that are taking place, if you are trying to understand decompression procedures. The deco software will give you limits on your ascent, with stops, in order to regulate the decompression (off-gassing) to within a tissue tolerance level, which is built into the software algorithm. After a given amount of time at the various stops, you may then proceed to a slightly shallower stop, in very small increments of 10 ft (3 meters) each. This drives the decompression procedure. Then switching to a richer mix of oxygen, which will also correspond to a leaner mix of inert gas, then the decompression speed at that stop increases, because there is less inert gas partial pressure across the membrane of your avioli compared with the partial pressure in your blood, which is washing all of your other body tissues (fat, muscle, and organs) clean of inert gas.
Remember that the inert helium will act independently of the inert nitrogen, even though each of these considered as a whole is inert. By varying the fraction of each, in your bottom mix, and in each of your deco mixes, you can also speed up their decompression. Your deeper mixes will contain more helium, whereas your shallow mixes will contain more nitrogen, and ultimately you will end up on pure O2, with no inert gases.
That is how it seems to work. Our research data result from goats in compression chambers, navy sailors, and cave divers. This then gives rise to complex mathematical algorithms, which ends up in your favorite deco software and/or deco computer. I use the DiveRite NitekHE as my primary with V-Planner generated deco tables as a backup with a gauge-mode SUUNTO. Because as ScaredSilly mentioned, you ALWAYS need a backup, for everything that your life depends on.
