Set up a short step platform slightly in front of a racked barbell.
Attach bands to each end of the bar and to the band pegs at the base of the rack.
Start with both your feet on the step and place the bar in a high bar position on your back. (Make sure the box is positioned so that it affords you clearance of the rack when the barbell is on top of your shoulders.)
Alternate legs in a reverse lunge pattern and explode through the top of every rep.
You see it happen all the time. An out-of-shape actor gets a part playing a superhero or action hero in a movie. A few months later, he's buff. You think, "Dammit, why can't I get into shape that fast?!"