look at frame 6, 7, 8, and 9. His head is mostly square with this shoulders. He could rotate his head a lot more to the right.
We both agree that the head is an indicator of what is happening elsewhere. It is meant that they shouldn't come up and out of the shot prematurely. If there head is coming up and around early it is due to incorrect form. Could the wording of "keep your head down" be worded differently, sure, but the idea still stands. I don't know of anyone who is suggesting people should actively prevent head movement, it isn't meant literally. We are constantly saying things in the English language that are not meant to be taken literally, but figuratively. When someone says keep your head down, there are trying to convey the idea of: 'don't let you head come up prematurely