The supine what? Yes read it again. You may have never heard of this exercise, but as you can see, this is actually a combination of 2 exercises. I love this exercise and I do this exercise regularly because it addresses 5 things: upper body flexibility, upper body strength, shoulder joint stability, core strength, and an increased metabolic effect. I’ll begin by describing the exercise start postitions and break it down, bit by bit.
To start, I lie in a supine (face up on my back) ‘table-top position’ on a workout bench with a dumbbell in each hand. I begin the action by very slowly lowering the dumbbells in a controlled manner into a chest fly position. From there, I will hold this position for 10 seconds. Holding this position, is in my experience, the most effective and aggressive way to stretch out your chest muscles. Most individuals have tight pecs and rounded shoulders from sitting at a desk all day and this section of the exercise alone is an amazing way to stretch the chest (and feels great too!). As I hold the dumbbells and let my chest stretch out in this deep fly position with my legs up in the table top position, I am simultaneously performing 10 alternating ‘leg drops’. This action forces the deep core muscles to stabilize the spine.
Once I have performed the 10 alternating leg drops, I return my legs to the table top position and then I begin to perform 5 very slow controlled chest flies, which builds strength and a fantastic isolation awareness of the pectoralis major muscles (the chest-see my previous blog post on Corrective Exercise). Once I have completed my 5 chest fly reps, I will return the beginning and repeat this sequence it all over again until I have done it for at least 1 minute. My favourite benefit of this whole exercise is that each time I lower the dumbbells into the deep fly position, I can feel my chest muscles stretch out deeper and deeper. By the time the exercise is over, my chest has gone through a very deep, profound stretch and I experience a feeling of being ‘opened up’.
WARNING TO THOSE WHO WANT TO TRY THIS EXERCISE: As with any exercise, this sequence has its elements of risk! Remember to begin by performing just the chest fly with a light weight first. The deeper and heavier you go into the fly, the higher the risk of injury in the shoulder joint. If you feel any part of your shoulder joint ‘kicking in’ then you are performing this exercise ineffectively- make sure your chest muscles are taking 100% of the load during this sequence. This exercise is complex. So just as with learning any new skill, it is important to begin by performing simple movements (correctly) first. Once you have a handle on the simple dumbbell movement (in isolation) then practice the leg drops (in isolation). Once you have correctly mastered the 2 movements, you can then combine the 2 exercises. Crawl before you walk kids 
Happy Training! 
Andrew Burchell
Advanced Health & Fitness Specialist