Sit on the ground with your knees bent and feet on the floor. Clasp your hands in front of your belly button. (To make this harder, you can hold a dumbbell in both hands, as shown.) Engage your core, and lean back a few inches to lift your feet off the floor. Be sure to keep your back straight; try not to let it round or arch in either direction.
Twist your torso to the right, reaching your hands toward the floor outside of your right hip.
Return to the center, then repeat on the opposite side, twisting your torso and hands to the left.
Continue for 30 seconds, alternating sides. Rest for 10 seconds, then do two more rounds of all five HIIT ab exercises.