Non-Fixable Freeflow: Modulating/Feathering the Tank Valve to Breath
The actual incident that happened:
Cozumel 2006, my adjustment knob assembly on the second stage regulator blew-out resulting in a catastrophic nonfixable free-flow, on a single-tank dive at a hard bottom 6m of depth, during a solo weightbelt lead check. In an a chance instant, I looked at the gauge pointer on the SPG and saw it rapidly sweeping from 200 through 150 bar, before quickly reaching back behind my head back to shut-off the cylinder valve. When the urge to breathe came, I cracked open the tank valve again to "sip" the torrent of air blasting out of the malfunctioning reg --and turned off the tank valve again. Switched out the malfunctioning reg for my bungied back-up reg around my neck and then proceeded to modulate/feather the tank valve behind my head on & off, taking breaths as needed, while slowly ascending to the surface.
The motivation after realising that you can breathe off a catastrophic non-fixable free-flowing 2nd stage reg, is to shut down the tank valve to stop further "hemorrhaging" of precious breathing gas. If you have a pre-dive Gas Plan usage and you know how much pressure your tank has during all phases of the dive, then why would you let vital breathing gas bubble away because of an unfixable free-flow? Especially if you have the ability to reach back and manipulate your tank valve on & off to take breaths while not panicking, still maintaining buoyancy at depth, spending a moment on the chance that your buddy will find you again? And if surfacing alone, then how about a nominal ascent with a safety stop while still modulating your tank valve as needed for breaths?
To recap on an Uncontrolled/Unfixable Free-Flow: Shut your tank valve down. When you need to take a breath, crack open the tank valve and shut it down again. Repeat as needed, switch to your back-up reg/octopus as well if the unregulated flow of gas from the malfunctioning primary reg is too much to handle. Perform this tank valve "feathering/modulation" technique whilst doing a CESA (if your buddy is nowhere to be seen and you're essentially solo). . .With your left hand, slow your CESA rate via BCD/wing hose deflator dump button . . .with your right hand reach back, feather/modulate your tank valve and take breaths as needed.
All it takes is practice (and although the technique is covered in tech & advanced overhead wreck/cave/ice diving training courses, IMHO --it should be a fundamental skill first taught in basic open water scuba . . ).
I think the issue is that, more-or-less, BSAC try to 'idiot-proof' the early training- a diver coming up on their buddy's air when their reg is malfunctioning is less complicated, thus less likely to be screwed up by an inexperienced diver. That, and there would be less temptation to continue the dive. (that, and it doesn't rely on you being able- under stressful conditions- to be able to reach behind you to turn your cylinder on and off.)
so yeah, not a bad idea, but probably not a good idea to train newbie divers to do it.
which also, thinking about it- using the kinked hose- has the advantage of being able to use your octopus- or backup reg- without having to worry about feathering the tank opening and closing. (it would also, it occurs to me, allow you- if you have a twinset with two connected cylinders that allows connecting a reg to both cylinders- to avoid needing to shut off a cylinder- instead of shutting off a cylinder- and isolating it- you could kink the hose- stopping the freeflow- then use another reg, while still having access to the remaining air in both cylinders,)
I must see about carrying something to stop a freeflow handsfree in future- even if I tend to stick close to my buddy, so i would be able to use their octopus, it would be useful to be able to minimise the lost air from a freeflow- if nothing else, it allows you to ascend to the surface on your own BC, rather than relying on your buddy's BC.