How to move a head from side to side with the mouse

There's a code you can type into the console that makes the mouse control only the characters head from side to side, vrs the default turning the whole character from side. We have an animation for both up/down and side to side that works with the mouse and have done this before (it doesn't trigger the animation, it moves the head along the animation path), problem is we no longer have the appropriate line of code to type into the console. Any ideas?

frank