The emissive property is part of the fragment shader program, which gets executed after the vertex shader program. In the UE4 material editor those aren’t distinguished to the user, a material consists of a vertex and fragment shader program. In which I don’t want to go into detail, but the vertex shader program get’s some input data and will be executed per vertex, it will then provide calculated data (from the input data) to the fragment shader program which will be executed per fragment (pixel).
I haven’t taken the logic behind this node apart, so I can’t give you any details on exactly how this node works, sorry about that. All you should know about the node is, it will not output the correct value at the point of the vertex shader program execution.