I think the positioning of the thumb is crucial for reach and accuracy of finger placement. I use the thumb behind the neck position for general playing, but will use it over the top when I bend or fret notes with it.
I also have the same DVD's by Jamie, and I am a big fan. I have found being aware of relaxtion at all times has really helped with moving up through the gears. I used to get a lot of tension in my biceps as I played quickly.
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