Stand in a staggered stance with one foot in front and your chest tall, fists up by your chin and your elbows against your ribs. You don't have to be near the boxing bag for this.
Lower your body and "duck" to the left, pivoting on your feet and keeping your chest up as you do. Come to standing.
With the same foot in front, lower your body and "duck" back to the right, pivoting on your feet and keeping your chest up as you do. Come to standing.
This counts as one rep. Keep alternating for 30 seconds.