Planar Shadows

I’ve been looking at the nvidia code for calculating a shadow matrix for planar shadowing and i’ve found it to be a bit confusing. Can someone explain to me how a shadow matrix for the stencil buffer needs to be created or show me where I can find a tutorial that will show me? (its obvious to me that it requires some matrix mathematics, but i don’t know what matrices to be used and what operations to perform on them).

(an obvious newbie to this opengl thing)