The bicycle crunch is one of the toughest abdominal exercises.  It is no wonder it ranked at the top of the list of best abdominal exercises in a study conducted by Biomechanics Lab at San Diego State University.  The bicycle crunch effectively involves the lower and upper abdominal muscles, as well as the obliques.

You will imitate riding a bicycle with your legs - hence the name of the exercise.  You could adjust the intensity of the exercise by the degree and direction of that motion.  It is toughest to do the bicycle crunch with your legs closer to the floor (going directly away from you).  It is easiest when instead of moving away, you move your legs up.  Another way to adjust the difficulty level is by how far you extend your legs.  Fully straightening your legs is much harder than moving your legs only slightly out. The most advanced alternative will be when your extended leg is completely straight  and roughly a foot off the ground (generally the lower you go the more difficult it is, but keeping a distance less than a foot leads to unnecessary lower back pressure).  As a side note, to make the exercise even more challenging, you could hold a dumbbell behind your head ( as illustrated in the video).

Since the exercise is difficult most people cheat.  They limit the range of motion, do the repetitions quickly by jerking their body, pull their neck, hold elbows in, and do not lift the shoulders off the ground.  These are all the things that you want to avoid. 

Neck soreness and pain are very common in performing the bicycle crunch.  Both are a direct result of incorrect form.  To avoid pulling your neck, only hold the tips of your hands on the back of your head.  Don’t overemphasize the movement of the neck.  Focus on turning your torso on each rep not your neck.  Looking at the ceiling as you perform the exercise may also help prevent you from excessive movement.

Just as with other crunches, in doing the bicycle crunch you want to ensure that you round your back.  Not rounding your back will lead to excessive tension in the lower back and could result in pain as well as injury.  Keep your elbows pointing out at all times.  You do not need to touch your knee with your elbow.  Maintain the distance between your chin and your chest at tennis ball's length.

The movement of your torso really consists of two movements.  First you will lift both your shoulders off the ground by rounding your back.  Second you will rotate your torso towards your knee.  Maintain good rhythm and breathe evenly (in one contraction and out on another).