animamation stops head movement unity

Source : https://answers.unity.com/questions/827590/head-rotation-while-animation-is-played.html
Answer by: Cherno.
//"You can/should be able to do it like this:
//Instantiate an empty gameobject at the neck bone's position.
//Make the head bone a child of this empty GO. The bone hierarchy 
//is now broken at the head, But that doesn't matter. Use 
//AddMixingTransform[1] to give any (preferably empty, meaning no 
//head movement taking place) animation clip the head bone
//transform. Play the animation just to be sure. The normal
//move etc. animations should work as usual until they reach the neck.
//It will probably require A LOT of trial and error, I know
//because I spend more than a week IIRC getting this to work.
//For my project, I wanted a character to get into a shooting
//pose, and then turn the torso towards the target and play the
//shooting animation for the torso only. It's a mess with 
//parenting/unparenting, Add/Remove MixingTransforms, Animation.
//Play and CrossFade... But in the end it works. Maybe you can 
//even use layers?"
//[1]: http://docs.unity3d.com/ScriptReference/AnimationState.AddMixingTransform.html