Start standing with your feet slightly further than shoulder-width apart, your chest up (your spine shouldn't be arched or rounded), holding the top of a dumbbell vertically at chest height.
Brace your core, and drive your hips back and down as you lower into a squat. Be sure to keep the dumbbell at chest height throughout the entire movement.
Rise back up by "pushing the floor away" and pushing through your heels, extending your legs to return to the starting position. Be sure to squeeze your glutes at the top of the movement.
This counts as one rep.
Perform this exercise for 40 seconds, then take 20 seconds of rest.