Great information and instruction in Laurent’s breakdown of the Fortnite system setup, its similar something I’m trying to do and helping me learn the UE4 animation capabilities fast.
One specific I’m having trouble with, that also wasn’t covered in the video presentation, was how to setup the upper body additive animations or its nodes(?). For example; I created an additive animation using a copy of my run forward. I used my weapon pose as the base pose. Hooking up this additive animation with the additive node in the animBP looks like it works fine, when used over the entire weapon pose. But, if I use a “Layered Blend per Bone” branching at the Spine (pelvis and legs playing locomotion animations as described) I get incorrect rotation of the Spine bone. It makes sense that its rotation is off cause the spine doesn’t have the rotation of the pelvis anymore, being that the spine bone no longer inherits rotation from any bone below it(pelvis). So, I feel like I’m missing something about the authoring process or the node setup. Should I create an animation that has extra rotation on the Spine bone to compensate for the missing inherited pelvis rotation? Is there a way to author additive animations so that I don’t need the “Layered Blend per Bone” to mask the lower body? Everything else is working great, I just can’t get these additive upper body animations to line up as expected. 
And advice would be most helpful. Thanks!