I think that the palm up technique illustrated above is a hold over of the correct hand position for a hip belay in which the rope goes around your hips rather than through a device.
If you are doing a hip belay, the stopping position is in the opposite direction from the stopping position when using a belay device and palms up provides the strongest grip because the hand gets turned over when braking.
In my opinion, with a belay device, the hand position needs to be reversed – eg. palm down.
http://www.traditionalmountaineering.or ... arness.htm
The movie clip recommended by Brenta is excellent and the same technique works just as well with an ATC or any other similar device.
http://www.dailymotion.com/video/x4x7q7 ... ppel_sport