Edge mask in Pixel shader (Fortnite GDC)

So I was watching the UE4 GDC talks and came across this in the Fortnite optimisation talk. It’s said that instead of doing edge outlines in post process, it’s done in the pixel shader.

How would someone achieve this? I didn’t really understand the description. I’ve been trying to do outlines without PP myself but I’ve had no luck.

I know a similar effect can be achieved with fresnel, but then whatever effect is getting blended, affects the whole mesh, not just the boundary.