Ok, I will try to explain why having the rest in the correct position is more forgiving. Basically it boils down to a simple lever. When you grip a bow you will torque it. I know of no one that can draw a bow and induce no torque to the grip. This will pivot the risor to the side, and in turn move the prongs of your rest. Since your arrow is connected to the string in the back and rests on the prongs of the rest it will pivot with the rest moving the point of the arrow from side to side.

Having the contact point of the arrow to the rest directly above your grip, or where the rotation is originating from will induce the least amount of movement in the arrow. If your rest is set up behind your grip (an overdraw) it will work as a lever and magnify the amount of movement at the tip of your arrow. So any inconsistancies in your grip will be magnified from shot to shot. This is why Overdraw rests are looked at as being less forgiving and should only be used by experienced archers.

An easy demonstration of this would be to lay an arrow on a table. Hold the arrow in place towards the front of the arrow. Move the back of the arrow from side to side one inch letting the arrow pivot where you are holding it. Now hold the arrow towards the back or in the middle and move the back from side to side one inch again. The tip of the arrow moves much more now than it did before.

This isn't exactly how an overdraw works, but it shows the effects pretty well.

Most drop aways are overdraw rests, but not much, maybe an inch or two and can be shot pretty well. Now the old overdraw rests used by 3-d shooters that were 4 or 5 inches back could be pretty tricky to shoot. And there is really no need any more with the faster bows and lighter arrows to choose from. Bascically an overdraw let you shoot a shorter arrow, which reduced weight. And since the arrow was shorter and stiffer you could get away with a lighter spine as well, which ususally means less weight too. Now a days if you want a lighter arrow, just pick a lighter arrow. You don't have to sacrifice forgiveness for speed.

I actually shoot a longer arrow than I need for the opposite reason. I have this thing for 2213 aluminum arrows (don't ask why?). So to get them to spine well with my set up I need to shoot them a little long. This also increases my weight which is ok with me too. Of course if I wanted more weight I could shoot a different size alunimum with a thicker wall.

Good luck, I hope I helped some and didn't confuse you too much.

Remember these are only opinions and nothing more. I don't claim to be a expert or anything.

Paul