Scott Robertson Matte Rendering Question

I'm not sure exactly where to post this, so if someone has a better place then just let me know and I'll move it accordingly.

For those who have seen Scott Robertson's "How to Render" DVD series, there's a point in the 1st DVD (rendering planar objects) where he uses a particular construction to determine at what point a light source will hit a plane in perspective. For visual reference, here's a link to show what I'm trying to explain:

https://lh4.googleusercontent.com/-i...r-surfaces.jpg

I don't really understand how he goes about constructing this, so if anyone knows a solid method to do so, I'd appreciate it. Any help would be great. Thanks!