What does "Head Leading" or "let your head lead" mean?
When you start a movement, there is one part of the body that moves first, that leads you. If you roll sideways, you can turn your head or move your leg to start the movement, for example.
When a back exercise talks about a body part leading (e.g. "head leading") it means that your head should be the first mover. But often it's quite subtle. Checking that another body part (e.g. your leg) has NOT moved first is often the best way to tell.