Avatar Mask Transform - does nothing?

So according to the official documentation:

"if you want more detailed control over which individual bones are masked off, you can use the Transform method to select or deselect portions of the model’s hierarchy to mask. "

Well that sounds great, but in reality checking or unchecking any/all/none of the boxes next to the bones in the list does absolutely nothing.

I’m trying to combine my existing walk animation with the top half of the body pushing on something so I don’t have to make a whole new animation for it. But I get the hips rotation from the lower half so the arms end up moving too. I should be able to disable the pelvis, it’s right there in the Transform list, but it does nothing.

So what have I missed or is it that this feature just doesn’t even work?

The transform mask only affects bones not controlled by the Humanoid rig.