In reply to Joel Perkin:
My first abseiling lesson was in 1971 with the Dulfer wrap-around; interestingly, down and over a bridge. I didn't see any trolls under it, they appeared much later. I did it without any safety rope from above and thought it to be dangerous and painful. But being ignorant of any other method I accepted it as being the standard. I was soon to run into more experienced novices who used twisted webbing around both thighs, a waist belt and the karabiner-brake bar. This was slightly less dangerous, but no less painful. Eventually I owned a Whillan's and a figure-8 descender. This seemed to be the ultimate abseil equipment because with a cross-over of the rope it could be locked-off, and I could hang, hands free --- sort of. The Stitch plate came along which served dual-purpose to be an abseil and a belay device. Then the refinements of this basic design improved it.
Over the last forty plus years I've learned a number of things which I employ as my general guide to going down rock faces and mountains.
One: don't abseil if it is possible to safely walk-off or down-climb --- both are usually much faster.
Two: If the party must abseil, do a check of who is most and least experienced and never be afraid to speak one's opinion of how it could be done. Take control if necessary.
Three: If the situation requires it, abandon anchor equipment. A back-up piece which costs 1/2 of one's hourly rate is .... you get it. Tie knots in the end of the rope if necessary.
Four: Concentrate, be aware, double-check, triple-check. The anticipated result of pulling knotted ropes can be the most nerve-wracking experience. "Please don't get stuck, please don't get stuck"
Onto the OP's question.
At this point in my evolution as a climber I use a BD ATC. I've used them for about 20 years or so. I belay with my left hand down, right hand up, and abseil the same way. It seems natural for me to do this, even though I'm right-handed. I use my left as the gripping (never-let-go) hand when playing ice hockey and cricket. I also use the same left-hand grip when frying eggs or holding a bowl to beat cream. I almost always use my right hand for fine motor skills. I like having my dominant right hand free to fiddle with things. So if cleaning or gardening a route, en-rappel, I prefer my right hand to be free.
I think a safety knot or Prussic should be used. I tie a 6mm cord above the ATC. It is not close to being long enough to be out of reach, less than 45cm (18in), so it cannot lock-up, out of reach. I can therefore use my upper right hand to easily move the knot as I slide down the rope. I prefer this to a lower safety knot which could get trapped in the ATC, especially unwanted during a free-space abseil. If the need arises for me to go "hands free" I can lock the prussic, then easily unlock it by doing a quick lift with both hands. This is very easy on less than vertical rock, less so in space, but quite do-able, by doing a leg (left) loop. If one gets in a pickle, use a second prussic and take all the weight off the ATC.
A couple of notes. If the person abseiling down second loses control, use a Fireman's Brake, i.e. pull the rope tight. It'll slow or stop them. Recently I climbed with friends who had new, very skinny, 8.2mm? 1/2 ropes. We did a 65m, partly free ab. My ATC was not supposed to be used for this. I tied a prussic, three times (not the usual two) around for safety because the ropes were so slippery and used an extra carabiner for friction. It worked perfectly. I began to learn how do to this on that bridge back in 1971. I knew there was a troll under it!
Be safe folks.
Post edited at 07:21