How do you create a damage shader that exposes endoskeleton upon taking damage? (video attached)

So I came across this video…I’m bit curious how did he do it? T-800 Terminator Damage Shader - YouTube

ive seen things like that a few times but dont remember the exact method. it could be as simple as vertex painting. or maybe painting to a custom texture mask.

Could it be something like mesh over mesh e.g meshA over meshB and upon damage meshA disappears exposing meshB