OK why do people move their Heads? Is it a "cause" in itself or an "effect?"
Mike O . . . Could you chime in here? I know you are an expert on the human head . . . . oh wait not ones that are attached to bodies.
Why do the head move?
Because it's attached to something that is moving.
If the top of the spine moves side to side or up and down, the head is probably going along for the ride. Most people refer to this as "moving the head", because the head is no longer occupying the same "space" it did at Address.
The head can also rotate sideways and up and down without the spine moving. However, this rotary movement isn't generally considered a swing "fault", because it can be done with the head more or less occupying the same "space" -- it just turns within that space.
One thing is certain: no one in the history of golf has kept their head perfectly within the same "space" from Address to Finish.
Threaded Mode
