In reply to iksander: I used these devices for a whole summer last year,
First of all remember the device is designed to add more friction than a stitch plate, not completely lock, as people often say!
I found that holding the dead rope and arc in the right hand and pulling the device up so it had the least amount of friction then pulling on the live rope the quickest and most efficient way while keeping saftey to an absolute maximum as it was centre based work.
I hope this helps,
We also spent time testing how well the device works in terms of 'semi-locking', we belayed several people of below, average and over weight people on a trapeze pole and had them jump off, with a group of instructors holding the dead rope at the bottom, and all loosening their grips on the dead rope, the rope actually slides through the device at a controlled speed that if you were to let go of the rope by accident then the climber would descend to the ground at a much lower pace than say a stitch plate