I agree with rdbass. The reason her rear foot is spinning as it is, is because of what she is doing (or not doing) with her front foot. She is essentially blocking some of her typical movements.'Squishing the bug' is spinning the rear foot(cause) to rotate the hips(effect).
I see this hitter's rear leg being the 'driver of this swing. I'm seeing the inner rear thigh being the 'driver' as the hitter turns the barrel.