like the guy above said before, it makes more mid to top end power, you retard the exhaust cam i believe like 1 degree (one tooth) and makes your YFZ450 have the same timing as the YZ450F, I helped my buddy do it and he said it had a lot more top end, and only sacrificed a very small amount of lower, it is the same as you advancing you cam like 3 degrees forward to try and gain some lower end on a warrior. They also claim it gives you 1-2 hp added and it is free to do, if you install the kick starter, you have to do it, not really sure why though....