Attach bands to the bar that are securely anchored near the ground. You may need to choke the bands to get adequate tension.
Use a box of an appropriate height for this exercise. Load the bar to a weight that still requires effort, but isn't so heavy that speed is compromised. Typically, that will be between 50-70% of your one rep max.
Position the bar on your upper back, shoulder blades retracted, back arched and everything tight head to toe. This will be the starting position.
Unrack the bar and position yourself in front of the box. Sit back with your hips until you are seated on the box, ensuring that you descend under control and don't crash onto the surface.
Pause briefly, and explode off of the box, extending through the hips and knees.