The seated box jump is a variation of the standard box jump that will help you build leg strength and explosiveness. Because you're jumping from a dead stop, it eliminates the ability for your body to use the stretch reflex and assistance of the counter movement.
You will need two boxes- or a box and bench/chair. The box you're jumping on should be taller than the box on which you are seated.
Place your smaller seat, box, or bench about a foot in front of the box on which you are to jump. Sit facing the tall box with your feel firmly planted on the floor and your knees bent at a 90 degree angle.
With as little movement or momentum as possible, press your feet hard into the ground and jump from the seated position onto the tall box. Land softly, but solidly, on the box with your feet in a stable position.
Stand to full extension on top of the box. This is one rep. Step back down off of the box and return to seated position.
You should feel very confident in performing standing box jumps before attempting the seated version. When you're first learning this movement, don't be afraid to use a small box and work your way up taller ones. Always be sure that your box is placed on a flat and sturdy surface to avoid tipping.