PREPARATION
1.Lie on your back, with your
  knees bent and feet flat on the
  floor.

2.Place your hands behind your
  head, elbows out to the side. 

3.Cross your left foot over your
  right knee. Breathe in.

EXERCISE
1.Breathe out as you raise your
  right shoulder toward your left
  knee. Keep your left elbow on
  the floor.

2.Hold briefly, then breathe in as
  you lower your shoulder back to
  the floor. Do not relax your
  abdominals between reps.

MAINLY FOR: External obliques