At the surface, a freediver starts their dive with proportional amounts of oxygen and CO2 with traces of nitrogen. During descent, you maintain the same percentage of gasses, but they become denser the deeper one descends. On comletion of the dive, you literally end your dive with the same volume of air that you had on your descent. The potential for DCS is almost nil. Once you breath off a tank, you introduce a fresh supply of air, which also then introduces the small percentage of nitrogen. Since it is compressed at depth, it is at a constant pressure - reascending, of course means the nitrogen will expand - you have become a scuba diver and can no longer freedive for the rest of the day. Freediving rules apply only as long as you do not breath off of a scuba tank - many of the dive physics and physiology laws apply to freediving, but in addition, some do not. This isn't something that can be explained in a public forum - one needs to have a qualified freedive instructor teach this material - suffice to say - dont breathe off a tank and freedive - you are setting yourself up for getting bent severly.