I'd like more information about correct thumb position. The beginner lessons don't really talk at about it but reading other stuff and through my own experiementation it seems pretty critical. It looks like the teacher is muting the low E on the Am and C chords and I've been practicing doing this as well as copying her thumb position (thumb visible) and it seems to help a lot. Are there any extra tutuorials or info on what's going on (or should be going on) with the thumb behind the neck?