Student or Learner
However, without having the hands pushing down on the head, the neck extensor stretch will be of a lower intensity than if it were done by itself.
--- Does it mean that if we don't put our hands on the head, but just lean forward, and pull the head down the stretch will be of a lower intensity?
This exercise can be found on page 11 here: Stretching Anatomy
It means the exercise will be better (higher intensity) if it's done with the hands pushing down on the head - that is, if it's done by itself as a single exercise, (as already described, I would guess).
It will be of lower intensity if you combine it with the other exercises that prevent you from using your hands in this way.