Why yaw to roll

Hey guys.I want a character to look at me but move only its head.I found how i can do this but i have a question.Why do i have to connect z to x and not z to z?This happens only in the animation blueprint.

my guess is because that may be how the neck bone is oriented.
Unreal assumes everything is built with X being the forward facing axis, and that might apply to
bones in skeletal meshes as well. so if that is the case (and you can check this by examining the skeleton) then probably the bone in the next facing forward toward the next bone in the head would be pointing up at the head along the bone’s x axis so rotating that would twist the neck side to side.

just a guess as i am not on my computer to try it out myself.