Yep, using o2. 100% oxygen. Think about it: you decompress to get rid of the n2 in your body's tissues. If you breathe air or any concentration of nitrox less than 100% o2, you are still adding n2 to your body. If you are trying to get rid of all of your old PC stuff, would you buy 100% mac or would you buy 21% mac and 78% pc? It'd take forever to get rid of your PC stuff if you keep buying 78% pc. So you switch to 100% mac and fwham before you know it you don't have any more BSoD in your life !
So when I reach 20' I'll switch to o2, cutting my deco time significantly. For example, the other day, the switch cut my deco obligation by 2/3. Makes a big difference.
Trimix is not being used because I have no worries about the narcotic potential of the gas at only 20 feet--you use trimix to replace the nitrogen in the breathing gas to avoid nitrogen narcosis at extreme depths, generally you try to keep the "equivalent narcotic depth" to 100 feet or so. o2 has no worries with END because it has no nitrogen, but you cannot breathe o2 below 20 feet very safely, as a Po2 of 1.6 is considered the maximum limit for most divers. Pushing it further significantly increases the risk of an oxygen toxicity hit.
So yes I do breathe oxygen on my deco stop, but the doubles I use for my breathing gas on the working portion of my dive contain air (or sometimes EAN32 if I've got the money for the stuff
), not pure o2.