Dosn't it seem in the Bustos video that she is super close to heel plant if not actually at heel plant before she starts the backside. What do others see. This ties into the thread about weight transfer as well.
I think you will find that the vast majority of slow motion video of the best hitters will show the back heel coming up off the ground as the front heel drops. The back heel coming off the ground is an indication that the back hip has started to turn/thrust. I know that's what I do when I swing, and also what Epstein teaches.
Epstein also teaches, that the hitter should invision a tight cord attached between the front heel and the back shoulder. Others may disagree, but I take this to mean that as the front heel plants, the shoulders start to lateral tilt and rotate. Said another way; when the front shoulder turns down and in as the hitter strides, the cord gets tightened to the max. As the front heel plants the back shoulder gets pulled down/back (lateral tilt) and shoulder rotation begins.
I have learned not to get to caught up on front heel plant as a check point for the swing. Only because the better hitters like Bonds, are able to plant their front heel and delay unloading their swing by "sitting", when they are early. As long as the hitter maintains the load in the backside, they should be ok.
I just started working with my 13 year old daughter on the "sit" technique, as she is starting to see more pitchers with the ability to vary the speed of their pitches.