You're arrow is getting a cleaner release from the string. W/out the STS, your string actually goes a little beyond brace, and then moves rearward (toward the shooter) to return to brace.....The tendancy is for the nock to have some rearward resistance on it at the point of release, again because of the string oscillation.....

BUT, when you install the STS CORRECTLY, your string comes to a dead stop at brace, and the arrow "snaps" off the string. A much cleaner release thus a couple feet per second faster......

I had that in my head, hope that makes sense the way I explained it...