Exclude bones from Mechanim

The main way to use an Avatar Mask is to assign it to a layer in the Animator window.

Note, though, that if your hands are children of the arms, and you're animating the arms, then they'll still cause the hands to move in space. If you need the hands to be in an actual fixed position, you either need to animate them like that, or perhaps look into using IK to hold them in place through code.

 

Ah, if you've set the mask up there then that's OK, but it's only going to be applied to that animation. (If you wanted it for everything the character does, it'd be tedious to set this up for each animation, so creating a separate mask asset and plugging it into the Animator would be easier).

 

It's in the Animation Layers section in the corner of the Animator window: http://docs.unity3d.com/Manual/AnimationLayers.html