According to a recent article on ab training, the supine leg rotation exercise is used to train the lower abdominals. Directions tell you to, “lie face up on the floor with your arms stretched outward in a straight line, palms down, alongside the body. Bend your knees to 90 degrees with your shins parallel to the floor. Then rotate the legs to the right and then to the left as far as possible without losing contact of the shoulders to the floor.”
This is a partial description of the reverse trunk twist exercise since it is the hips not the legs that are rotated in this exercise. The legs are in motion but the muscles involved turn the hips not the legs. When you turn the hips-legs as far as possible to each side you effectively involve the obliques and not the lower abs although some of the obliques do cross the lower abdomen.
The key role of the lower abdominals is to stabilize the pelvis so that it can move more effectively when the obliques contract. Thus they remain under static contraction to hold the pelvic girdle stable with the thighs in a perpendicular position.
Also, when you keep the arms alongside your body you limit the range of motion greatly. If you go full range you will roll over. As result, not only do you not get maximum muscle development, and but you do not even get a good stretch of the lower back which is another great benefit from doing the reverse trunk twist.
See Kinesiology of Exercise for more information on this great abdominal exercise targeting the obliques. It is one of the two best exercises for isolating these muscles of the abdominal group.