The bicycle crunch is traditionally used to work the oblique muscles. While that may be true, this exercise is good for overall abdominal strengthening. The rectus abdominis (the "six pack" portion of the abs) is probably more active than are the obliques. Nevertheless, it is still an effective ab exercise. Instructions for the bicycle crunch are listed below:

Lie down on a workout mat. Interlace your fingers behind your head. The elbows should point outward. Lift your legs off the ground with knees bent. Next, rotate your torso as if to point your elbow toward the opposite knee. As you do so, bring the knee in toward your elbow. Your legs should move back and forth as if you are riding a bike. The torso should rotate in concert with the knee on the opposite side of the body. Repeat the movement in alternating fashion.

IMPORTANT: Be careful not to pull on the neck, as it can compromise the integrity of the spine. Perform this exercise in a slow and controlled manner. Try to focus on abdominal contraction and not so much the movement of the legs and elbows.