Fortnite Shadow Fakery?

yeah this is not at all what my question said, I’m literally only talking about the trees using a vertex shader to simulate long distance shadows by moving two vertices of a triangle plane in relation to the sun. Looking for an explanation on how to manipulate a mesh like that with shader logic, I’m an artist not a programmer

Edit: Unreal Engine - Live from GDC This is where he explains tree imposters, and “Shadow Fakery”