Train repetetively the motion you are attempting to gain speed with biomechanic perfection and lose as much bodyfat as possible. It only creates inertia for your muscles to overcome.
I suggest explosively forcing the weight upwards on the positive portion of the rep but move slowly on the negative portion so you do not injure yourself. This helps to train your nervous system and creates more explosive body movements. We used to train this way during football. I also recommend plyometric training. This will drastically increase your vertical as well as reduce your 40 yard dash time. Those jump soles are also good for adding to your vertical and shaving time off of your 40.