On a smooth surface, start in a lunge position with the right leg in front and left leg behind, foot on a towel. Squat down and hold this position with the front leg, making sure the knee does not go forward beyond the toes. Slide the left foot back until the leg is almost straight (but not locked), then slide the left foot back in until the left knee is directly below the left hip. Keep this movement going on a 4-count-in, 4-count-out basis. Do one 2-minute set, then repeat on the opposite side.
To mimic pulling the weight of the Supra carriage, add 2.5-pound ankle weights and slip a resistance band around your legs. Place the band right below the front knee, and above the back of your rear knee.