Hi there Glyn,
Sorry, just saw this thread now... The exercise that you're describing is something I practice very regularly as part of my stretching regimen for freediving. I find it the single most difficult and also beneficial stretch that I've tried to date. The difficulty is all mental for me, but let me describe.
I'll describe how I do the diaphragm stretch, but do note that I may not be doing it "correctly."
1-I start by sitting cross-legged on the mat. After relaxing and doing a few breathing patterns I bend forward at the waist till my head is touching the floor while simultaneously exhaling as much air as I can.
2-Without inhaling at all (keeping lungs as empty as possible) I sit up with my back as straight as possible with my hands on my knees. In this position I attempt to inhale against my closed mouth/epiglottis. At this point you'll feel your belly start to move back. Keep pulling and you'll start to feel a stretch in the diaphragm. I try to envision pulling my bellybutton up behind my ribcage.
3- Still holding, I repeat the stretch as many times as possible, until the desire to breath is overwhelming.
4-During repeats, I reverse pack an increasing number of times during the exhale.
During these stretches, I find the desire to breathe is almost immediate and extremely strong due (I believe) to the negative pressure in the lungs. It takes a fair bit of mental control (for me anyway!) to hold the position and repeat the stretch several times. That said however, immediately following, I feel an incredible relaxation and a feeling of amazing clarity of thought. Because of that, I use it as the final stretch in my regimen before entering the water.
If you decide to give these stretches a try, start gently. I remember being a bit sore after an overzealous session!
The description I've just given is what I've arrived at on my own from reading books, watching others etc. If anyone has anything else to add PLEASE do! I'd appreciate it as, I'm sure, would others.
Happy dives,
Aaron
