Plyometrics exercise's with jump training not only makes you faster, but it also increases your strength which benefits your explosiveness. Helping you increase your fast twitch muscle fibres can benefit not only your speed but also strength and size. The goal is to use platforms or boxes of varying heights and jump using a variety of speeds so that you can work your body in a number of ways. This will promote both strength and muscular gains.
If you find the height too easy then you can increase the resistance by adding a weighted vest into the movement. Adding an extra weight on to your upper body will make your legs compensate adding more balance and increase the muscle fibres activated in the jumping movement.
Also another way to increase the resistance in this exercise is by adding more movement, such like lateral jump or placing a twist into the exercise. Beware this should only be tried by an advanced experienced trainer, as these movements take a lot of skill and practice. If you do wish to try, I strongly recommend that you only use a small step to start, before you attempt the box jumps. First get used to the movement and gradually progress the heights.