Honestly, I agree that some changes in our workouts are always welcome, but in my humble opinion this is one of the more strange ways to do curls! Here is some similar exercises, which are not as awkward:
I agree with Intensity, this particular exercise you are talking about is awkward at best, and downright uncomfortable at it's worst. You could use it every once in a blue moon just to try to get a different angle on your bi training, but I certainly wouldn't do it any more than that. As for the correct most "comfortable" hand placement, the grip should be exactly shoulder width (in other words, your hands lined up with your front delts) and make sure to only move your arms when you squeeze backwards. If you start to move your shoulders then the exercise is a total washout, Vin.
"If the shoe fits, kick em' in the teeth with it!"*** Bobby "The Brain" Heenan