Yeah, I don't know. A few things are obvious, and I've witnessed them many times in my self, and others. I'll take the non-scientific approach (as there are people here with much stronger background in the scientific), and just go with the gut feeling and personal account
-Submersion in water slows the heart. The colder and deeper, the stronger the effect. Combined with apnea, it is very strong
-Same effect to lesser degree with dry apnea
-The emptier the lungs, the stronger the effect (both dry and wet)
Heck, I'll throw in a graph from my heart rate monitor. This was a dive done without any preparation (as you can see from the HR of 130, I had just swam to the buoy), with FRC to about 10 meters in 4c water. If you look at the time scale, the drop is imminent. From 130 to around 40 in 10 or so seconds!
I think this, among other things, is partly simply a mechanical result of lower pressure in the lungs -> higher blood volume -> less pumping
Packing is really counter productive in terms of all this, it more or less eliminates the effect.
When I "experience" MDR, it is almost always with FRC or "not completely full" lungs.
But, all that said, these facts remain:
-My best perfomances are done packing to the point of uncomfortable and relatively high heart rate (in the 70-80s vs 40-50s without packing)
-I still perform better in apnea on dry land
-Exception being deep diving, for which there is no way (?) to compare to dry apnea results, but there definately is a "jump" in performance (for example a 2 minute dynamic in the pool is torture, but 2-3 minutes in cw goes by like you don't even notice).
Personally, I believe the 2 biggest factors in "MDR" to be the breath hold and pressure. In fact I usually call it "apnea reflex". I don't think there much debate weather or not it exists...Facial immersion etc may have some effect, but at least for me it is negliable. Sure I feel nicer doing statics face in water, it may even show in the HRM, but the objective truth is, the times are not getting any longer. Maybe there are strong personal differences? But anyway, for me it seems that in pool conditions, it doesn't really "happen" with just floating face down. But with empty lung dives to 4m I sure can invoke some pretty cool HRM graphs
For me, personally, MDR is a series of "tricks" I do to get "into the zone". Which means this strange, slowed down, state which feels almost like being a little drunk. During that state I can do long dives and be very comfortable. Never really feeling the need to come up. But, I've noticed, it doesn't seem to affect (at least very dramatically) the absolute performace. I can crank out just as long or deep dives without it and feeling like crap...It's just that the former method is much more fun!
