The medial heel whip is also called an abductory twist. This occurs when the heel suddenly ‘whips’ medially or twists in an abductory motion just as the heel is coming off the ground during gait in an ‘overpronated’ foot. This video describes it very well:

While the video gives one explanation, there is some controversy as to if the problem is really located in the foot or due to more proximal problems around the hip. There is a good discussion of this on Podiatry Arena and some comments in a French forum (use Google translate).