What is a double radius shelf and what does it do?

If you look at a properly made shelf it should be radiused both from front to back of the bow AND from the edge of the sight window out to the outer edge of the bow, hence, double radiused. A shelf should not be flat for shooting off of it. In this way, the arrow actually rests only upon a 'point' no matter where your nock point ends up on your string.  Hope that helps!

Excellent explanation by Plainsman   .

Thanks, I just wasn't sure what the double part meant but I've got it now.