Fallen Cross is one of the perfect oblique exercises. Since your body is parallel to the ground it focuses attention directly onto your obliques. This exercise is designed to also increase your balance. The practicality in terms of increasing oblique strength is that you are engaging your stabilizer muscle groups.
How to do a Fallen Cross:
In order to perform this oblique exercise, place one hand on the ground with your feet straight out. Your body will be perpendicular to the ground. Lift your other hand high into the air and look toward that hand. This will help increase your balance and put your body in the proper form. Hold the position. When you are finished on one side, switch to the other side.
Muscle group worked:
Target Repetitions for Muscle Growth: this oblique exercise is based on seconds held, not repetitions.
Target Repetitions for Muscle Tone/Endurance: this oblique exercise is based on seconds held, not repetitions.
Why the Fallen Cross works:
The key to this exercise is the position of your body. Since your body is perpendicular to the ground, it provides a direct impact on your oblique muscle groups.
Tighten both your left and right sides of your obliques as you do this exercise. This will engage even your top obliques. It is still important to switch sides though the extra tightening will help fatigue the muscle more effectively.
Make sure to breathe slow and steady throughout the workout.
Disclaimer: Talk to your primary care physician before beginning any exercise regimen.