This is exclusive to MLTA. I've used this trick for years but never written it up before. If you are not getting enough friction from your abseil device, perhaps (like in this shot abseiling on a 9mm rope), here's what you can do to solve the problem. All you need is 2 karabiners. Clip the the krab to your abseil loop and clip the brake end of the rope through it. Now pull the brake rope upwards (this will already create a much better grip) and clip the 2nd krab onto both the brake rope and the main abseil line. Now holding the brake rope in the normal abseil position you will have plenty of friction. Screwgates are preferable but if necessary you can scavenge a couple of karabiners from your quickdraws as in this picture, taken during a genuine incident -abseiling down to the Britomartis belay at Wen Zawn, Gogarth.