-Laying on the back, lift your straightened legs from the ground at a 45 degree angle. 

-As your Left foot travels downward and nearly touches the floor, your Right foot should seek to reach a 90 degree angle, or as close to one as possible.

-Bring your R foot down until it nearly touches the floor, and bring your L foot upwards.  Maintain leg rigidity throughout the exercise.  Your head should stay off the ground, supported by tightened upper abdominals.

-(L up R down, L down R up, x2)  ^v, v^, ^v, v^ = 1 rep

-Primarily works the Rectus Abdominus, the hip flexors and the lower back. Secondarily works the Obliques.  Emphasis placed on the lower quadrant of the abs.

The diagram shows the most used muscles on this exercise