I’m getting closer to resolving both issues of the head stability (shift or no shift?) and the trailing heel impact position, and I made a link today between the two, believe it or not.
It’s all due to the way the body is built to move and, from where I can see things at the moment, there is a direct link between having a flat or low trailing heel at impact versus one that is higher and whether or not your head shifts on the back pivot.