what is trapping?
Usually the chamfers are much bigger than shown in MM's drawing. It is called trapping because you make the limb cross section into a trapezoid (or close to it). The attached picture shows two ways to trap a bow limb. The wider side of the section is the belly side.
Fibreglass bowyers and wood bowyers tend to use it for different reasons. As MM says, it reduces limb stiffness. FG bowyers use it to drop weight when a bow turns out much heavier than intended. Wood bowyers use it to reduce stresses on the belly and increase them on the back. This is done because wood typically fails in compression well before it fails in tension. Trapping helps to balance the stresses so that the belly doesn't fail well before the back.
Mark