I’m not a programmer so I can’t tell you how it’s coded and I imagine it’s completely different in UE4, but I know how it functioned in Source engine which is why I mentioned it. Load up a game like Left 4 Dead - shine your flashlight on a piece of furniture and then walk around it. It’s just another version of the mesh that’s textured black and transparent and then the flashlight is creating a pivot offset of some kind. I imagine that dynamic shadows in UE4 use a similar technique.