Medicine balls are fantastic for athletes to build strength. Medicine balls can be used in all planes of motion and engage muscles from all areas of the body.
Coming out of your stance like a rocket it what it is all about. Vertical push is critical to defeating a defensive back. The ability to vaporize a defensive back and leave him to spontaneously combust is just plain fun. One of the goals for my athletes is to cover 10 yards as fast as possible. When you are locked up one-on-one and you look across the line of scrimmage, eyeball your competition. When both of you make eye contact make the defensive back blink first.
You can win the battle of the line of scrimmage before the snap. How? First, get in your stance and look like an overwhelming force. Make the DB think he will be crushed because he is in your path. Look fast even if you are not. Secondly, when the football is snapped, open it up, detonate, erupt, and come out like a rocket. Let the DB know you will compete on every snap.