How the playable graph work without avatar mask in 3rd person hero animation

the 3rd person hero animation is fully driven by the playable graph that I can check with playablegraph visualizer.but i dont understand how the "avatar mask" work.I have not find any references of the void
SetLayerMaskFromAvatarMask of AnimationLayerMixer,I can not find find any useage of AvatarMask except the ModuleImport;
below is the avatar mask that all 3rd person terraformer's clips use.No other avatar mask only toggle the leg skeleton or the body skeleton.

how the mask animations between the body and the legs work? For Example,when the character went forward and shoot something at the same time,the leg animation is run and the body animation is shoot,if there no avatar mask,the leg animation will be affected by shoot animation,so i wonder how the playable graph work without avatar mask