While most plyometrics take advantage of the stretch-shortening cycle to produce power, the seated box jump removes all eccentric loading and allows you to focus on only the explosive, concentric action of the movement.
Taking out the swing of the arms will force you to focus on developing power from the ground up. To take this movement to the next level, hug a weight to your chest. Now you have a loaded plyometric movement that doesn't trash your joints. Not too shabby.
Like any box jump, make sure you're truly able to land on the box to which you're jumping. Choosing a box that's too high doesn't make you more of a man, though it will remove some flesh from your shins when you miss.