This is something that I have been seeing for years and did not understand the significance of it! When some people are walking or running, just as the heel comes off the ground, there is a slight whip or movement of the heel inwards. I now know that in Podiatry, the call this an abductory twist and with physical therapy they call this a medial heel whip. It is caused by the sudden loss of friction when the heel comes off the ground and the foot is overpronated. Here is a neat video that explains it:

http://www.whatispodiatry.com/questions/what-is-an-abductory-twist/