By adding weight to the ends of the string, you offset the weight near the middle and reduce vibration. It is no secret that this extra weight will add more speed to your set-up. Try it and see for yourself. Basic nock sets added to ech end will do. Thats what bow companies use on their string, standard nock sets with shrink tubing around 'em. I really do like the Spyder Speeds, they do not seem to add speed to my particular set up when used with the Spyders BUT I do not loose any as well. Speed loos is suffered from any additional weight you normally add to the string . Now when the speed balls are used by themselves, they do indeed increase your speed. I have not seen a huge gain like 15 fps but I have seen 5 to 7fps on many set-ups.