Question "Override" blending ignores AvatarMask

Hi all,

I've been trying to override my right arm's animation. To do this, I've got a layer called "Attack" with an AvatarMask masking everything but the right arm. When the blending mode is set to "Additive" it works as expected (though the animation looks stupid, he just kind of twitches his arm).

To fix this, I thought that setting the blend mode to "override" would override the right arm idle animation and replace it with the animation for the right arm attack (i.e. the places that AREN'T masked off will play the attack animation whereas the rest of the bones will still follow the idle animation). However, when "override" blending mode is selected, it ignores the mask entirely and just plays the entire attack animation.

Surely this is possible!

I'm likely X/Y probleming myself here, so to be clear my goal is to replace the right arm of an idle animation with the right arm of an attack animation when a certain function is called.

Apologies if this is a newbie question but I'm extremely new to Unity animation so I don't have the vocabulary to properly formulate the question I want the answer to.

Thank you!