Check out the disc mania video on youtube where Avery talks about backhands.
Near the end he suggests pivoting off the heel then demonstrates it clearly on the video.
Amusingly however, in every drive up to that point in the video he is pivoting on the ball of his foot.
It is a very good video though. Check it out.
http://www.youtube.com/watch?v=rCs5u9ecVis