I believe it is a high volume of air.
I am not going to look of the exact formulas, but basically consider this.
If you have 10 cubic feet of air, at 90 PSI, allowing that amount of air to expand until you have 45 PSI, that amount of air will now take up around 20 cubic feet.
I am ignoring temperature changes, and I do not have the exact formula, and there is probably other errors because atmospheric air pressure is about 15 PSI.
But the point is that if you have a fairly small volume of high pressure air at a high pressure, you can get a larger volume of air at a lower pressure.
I use this spray gun. The manual that came with the spray gun says it requires about 9 SCFM at 40 PSI. The dual tank setup I described in the other thread allows me to run this gun, with a 3 HP compressor, and the compressor cycles on and off, typically with a longer off time than an on time.