I agree with ol hunter except for one thing. When your set up is not tuned properly you lose speed and, depending on yardage to your target, penetration. If your arrow is porpoising, (up and down) or fishtailing, (side to side) your speed and penetration will suffer substantially. Say your at 20 yards and you have a porpoising problem. If you nock end is on the upward travel when it impacts the target, approximately 45% of your kinetic energy is directed to the nock end. Walla, poor penetration. On longer shots the arrow has time to correct with the fletching stabilizing the shot. Did you ever hear one of your buddies say: I was only a 15 yard shot but it didn't pass through the deer" This is why! If you have the time, do it right and you'll not only have a tuned setup, but your confidence with be as tuned as your bow.
Be Safe Where a harness.