This how-to shows a great muscle combo workout: legs and abs. Men's Health, the preeminent source for fitness videos, breaks it down for you.
Loop resistance tubing around your ankles and pull it up your legs until the loop is just above your knees. Place your feet more than shoulder width apart and squat slightly so you feel the bands' tension pressing against the outside of your lower thighs.
Keeping your right leg stationary and both feet flat on the floor rotate your left knee inward then out, for the prescribed number of reps. After finishing with your left leg apply the same principles to your right leg and match the number of required reps.
As with any shoulder or core exercises, adding rotation to your leg workout helps stress muscles often neglected in standard exercises. To advance, try using stronger bands to increase the level of resistance.