When you add an overdraw your bow becomes harder to shoot accurately. This is going to be an extreme example but you will get the idea. Immagine that you have a 2 foot overdraw. When you torque the bow one way or the other you don't see much movement at the grip area but at the end of the overdraw you will see a very noticeable movement left or right. When you shoot the arrow your rest will move more at the shot the longer the overdraw is therefore your arrow moves left or right.