How to tweak action strip in NLA?


I have action strips in the NLA and I want to change the position of one bone.

For example, I have a character moving his head around. That’s my action strip. There are a few frames where his head is clipping through his body, and I want to go in and change his head’s position/rotation. I want to do this without having to fix the original animation strip.

I know there’s an additive layer feature? I don’t know how to use it.

Any help would be much appreciated!