It allows you to make a mistake without negitive results. It has to do with the length of time the arrow is engaged and on the rest. The shorter the time the more forgiving a bow is said to be. But, the shorter the time the less push on the arrow you have (speed loss). You can buy a bow with a long brace and keep it that way or shorten it with an overdraw (to see what works for you) but, you can't buy a bow with a short brace and make it long.

It's just math. Plug in the numbers and see what they are based on your spec's.