Get shadow buffer of one light source to a material

Hi, in the recent Unreal Engine Livestream, the one about hair shaders in Paragon, https://www.youtube.com/watch?v=toLJh5nnJA8, Brian explains that he gets the information about a shadow (mentioned around 22 min), and uses this information in his shader.

I want to do something similar, where can I get this shadow information?