Feathering weights on the spine

I'm trying to figure out how to feather the weights on the spine bones in Unity to use an upper body motion like a gun reload while moving. I'm using Animation Type Human and I thought it would be in the Animation tab under Mask > Humanoid but there doesn't seem to be a spine bone selector there.

Any ideas?

 

To do this kind of setup you need two layer in your controller.
The first layer control the whole character.
The second layer should have an avatar mask with only upper body part enabled and should take care reloading.

 

It doesn't let you select each spine bone in the avatar mask from what I see.. For example with a walking reload I'd like Spine (base of the spine) to be at 100% of the walk anim, Spine2 to be at 50% walk / reload and spine3 to be at 100% of the reload.

If there's no spine feathering when you play a separate upper body animation while while walking / running your torso is going to wobble because it's parented to the root and no longer counter animating the hips/roots rotation.