What is actually done to the bow when the bowyer reduces the weight?
I believe they sand the limbs down and it gradually reduces the draw weight.
The limb edges?
That im not sure about, if you ask this on the bowyers bench you will get a detailed answer for sure
Depends on how much weight has to be taken off and the style of the bow to begin with. On a wide limbed recurve, working the edges works pretty good. Sanding the back or belly takes off weight very quickly. On a deep cored longbow, sanding the back or belly, provided there's sufficient glass thickness to begin with, will do the job. Sanding the sides of the limbs doesn't have as much of an effect as soon. If a lot of weight needs to be taken off, folks sand both the sides and back and/or belly.
This is for a glass laminated bow, of course. Shouldn't touch the back of a self bow. Working on the sides or belly is about the same. Belly will take the weight down faster than working on the sides.