I think that having the proper brace height reduces vibration and noise from the string upon release. Twisting the string will increase brace height and make the string rounder. I'm sure someone here can correct me if I'm wrong.
Changing the BH on a recurve or longbow is similiar to moving a compound rest left or right. In other words, it affects how the arrow leaves the bow. When shooting fingers, the archers paradox causes the arrow to flex in a horizontal plane.
Adjusting the brace height, while bare shaft tuning, allows for optimum arrow flight. When it is set correctly you will have a "sweet spot" where noise and vibration are near minimum and the arrow flies the best.
As you probably know, BH is adjusted by twisting the string and can take a while to get it just right. Over or under twisting can be detrimental. Also, after twisting, your knocking point will need to be adjusted. Get the BH right first and then the knocking point. You'll need to go back and forth between the two for a while, but what the heck, it's fun to watch 'em fly.
If you've got a sharp eyed friend, have him stand behind you and watch the arrow flight while shooting at least 20 to 30 yards. Have him look for the knock or fletch moving from side to side. Twist until this movement is at the absolute minium.
Anything that changes dynamic spine (arrow length. point weight. fletches.wraps, crest, knocks,etc.) will cause a need to re-check
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