Double click the 3D Motion Layer track at frame 9 to insert a key.
Move to frame 10 and turn the head.
Move to frame 19 and double click the 3D Motion Layer track.
Move to frame 20 and turn the head back.
* Now the head will turn at frame 10 and frame 20.
What originally happened is that between frames 1 and 10 it was tweening the frames which means that it approximated/averaged the head turn between those frames so that there was a smooth transition. That is why the head turned earlier.
It would be the same as moving to frame 10 and placing an arm straight out.
When you play it back the arm does not suddenly jump straight out at frame 10 but will slowly raise the arm from frame 1 until it is straight out in frame 10.
Placing the extra key at frame 9 maintains the the same position as the previous key (at frame 1). Same for frame 19 where It maintains the head position from frame 10.
See my CTA Tutorials on the YouTube channel CTAStepByStep
Check out EZColors for CTA. The easy way to change Render Styled (RS) colors.