Took a day class from Joel Turner (elk calling champion) who is also a precision shooter with both archery and firearms (firearms instructor for SWAT). I thought I was a fairly good marksman with a bow until I went and heard his proven theories and techniques. Essentially, and to not make this post long, your body runs off a natural, uncontrollable brain-ran reaction (sub-conscious) that even when you are calm and slowly squeezing a trigger from a gun or bow your muscles move very slightly just before firing in a 'brace for impact' reaction...this is the 'jerking the trigger reaction that a lot of people are talking about. He proved this to us in the class by having us hold a bow in the shooting position with no arrow. When your brain tells you there's no chance for impact your stance and target pin-to-target hold is ROCK solid with very little movement because your brain is telling you theres no chance for firing. He then had us hold the position with arrow knocked and told us to ready to shoot...what was interesting is there was always drift and you found yourself constantly moving the pin back to the target in small movements, normal target movements.
To curve this you use your muscles to shoot/squeeze, thus taking away the part of the 'brace for impact' reaction when you sub-consciously tell your hand to 'shoot' or 'pull the trigger'. The steps are to go to full draw and set your anchor point, then set your finger on the trigger but DO NOT pull the trigger with your finger, instead slowly flex yours back while aiming. Your trigger needs to be set VERY lightly and part of the day class was setting releases to accommodate this. When you slowly flex muscles, your finger will actually pull the release and let the arrow fly, however you brain has NO idea when this is going to happen and the 'brace for impact' reaction is not inadvertently sent to the brain and last minute movement occurs. The hardest part of this is learning to NOT MOVE your finger and just use it as a fixture over the trigger and let your flex reaction do the work.
This is hard to describe, but it works!! He described the brain/reaction process so much better and has loads of scientific proof and in-field examples he shows you to prove it when you take his class. He actually videos you when you shoot and it proves that just about EVERYONE has last second movements that hinder precision marksmanship. I've completely changed my way of shooting and I know I am a much better shooter because of it. You should really check Joel Turner's methods out, his classes are called 'Iron Mind' and hes a hell of a nice guy and very accomplished hunter, and if your lucky he will show you some impressive elk calling techniques. HIGHLY, HIGHLY recommended.