Tim,
That is a subject that NO ONE can cover in one blurb. Books have been written about it.
Basically, it is a relationship of 3:5.
It applies in building in numerous ways. Example (Patch box, 3:5 with buttstock,)
No one is sure if it was truly applied by original builders, or if it is an "eye" thing.
MBS has a booklet that explains it simply, and with illustrations. Interesting concepts. I've applied in my own designs, it does work.