it dictates how long the arrow is on the string. With a shorter bh the arrow is on the string longer giving you more time to make form mistakes. Generally as the bows have progressed, bh is less important when it comes to forgiveness. some will debate.. so longer the bh, more forgiviness but less speed. shorter bh, less forgiveness but more speed.
Usually, shorter brace equals a faster arrow. Some claim that a taller brace equals a more forgiving bow. Forgiving is a. Rather vague term used mostly for advertising purposes though. In real world speak. A taller brace will be slower but will be less apt to slap your arm or hit a coat sleeve. A lower brace will shoot a little faster but will be more apt to hit your arm if your grip is off. My bow has an 8" brace and I like it a lot. I'm not interested in speed too much so that part doesn't bother me. I shoot mostly from a ground blind where I'm in a sitting position and my form isn't always as good as when I'm standing. The taller brace helps me to maintain accuracy somewhat when I'm not able to get that perfect T form.
A longer brace height is thought to make a bow more forgiving in terms of accuracy by how long the nock/arrow stays on the string which helps prevent human error but with as fast as bows are these days and the cam technology, it just doesn't seem to matter as much anymore. Mostly it helps provide a better draw cycle, but it really depends on what you're looking for in a bow. I say if you draw a bow that feels comfortable, whether the draw weight is 50 lbs or 70 lbs, if you can shoot accurately with it, that's what truly matters. Speed alone means nothing, accurate speed means everything. That's just my personal opinion.
Here's why it's important to me...I had both a 6" BH bow and a 7" brace height bow last year. I shot the 6" bh bow great in a t shirt...slapping arrows at 20+. I always towards season start practicing with all my hunting gear on. So I put my late season jacket which was some predator fleece at the time. I noticed I was getting string contact with the thicker sleeve on that jacket and my groups were opening up. I tried an arm gaurd for a while, but still got occasional contact. So I compared my 7" bow to my 6" bow and with hunting gear the 7" brace height wins. If I lived south of say Ohio it wouldn't matter cause its not as cold. Then I'd go 6" brace to get the Xtra speed. So for me for hunting bows its 7" brace heights all the way.
