Setup
Lay with your upper back supported against a plyobox, knees bend with hips, knees and second toe in a straight line, holding dumbbells in either hand with overhand grip with handle of the dumbbells resting in the crease of each hip.
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 in vertical position, 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