Setup
Lay with our upper back supported against a plyobox, knees bend with hips, knees and second toe in a straight line. Barbell resting in the fold of your hips with hands on the barbell.
Movement
Tuck your chin slightly, engage your core and lift your hips off the ground to drive the weight upwards while maintaining a straight back as you pivot your upper back on the plyobox. When shoulders, hips and knees are in a straight line, slowly drop down your hips down slightly out of the end position while your shins are still vertical, then return up to the start. You are basically completing the end range movement of the hip thrust exercise repeatedly.
Tips
Do not over arch our low back
Do not flare out your ribs as you lift