Samba
Your description of SWB is accurate enough.
Samba is a loss of motor control without a loss of consciousness. You shake or 'dance'.. hence Samba.
One can take it that in a case of SWB you dont make it to the surface, unless you float up with luck and good weighting.
With a Samba you obviously did make it to the surface by your own volition and propulsion but on reaching the surface you suffered a shake. This can degenerate into a blackout if severe enough.
One reason given was that "blast clearing" of the snorkle at the surface "almost invaribly leads to SWB if a diver is near there (sic) limit".
Normal repetitive diving should not bring one close to the
limits where you are on the verge of losing consciousness or a loss of motor control.
Many spearo's will spit out the snorkel before diving but not really for the reasons of avoiding having to blast clear it. They do this to flood it so to avoid bubbles trickling out and scaring the fish. These divers usually replace the snorkel on the ascent and release a small amount of air which expands on the ascent driving the water out and leaving the diver with an already cleared snorkel as they surface.
Other divers exhale on the way up in the last 5 - 3m. These guys believe they decrease the O2 'vacuuming' effect - for if there is no / less / little, air in the lungs it cannot suck much air from the blood- decreasing the possibility of SWB. Others do it to get the exhaling part over and done with and clear the snorkel for free with no effort, by the action of the escaping and expanding air.
As for a blast increasing lung pressure, it might briefly but its increasing pressure of stale air. Not much use to anyone. A hook breath on the other hand increases pressure of the good stuff. That good stuff incidently takes 10 - 15 seconds to hit the brain where its needed. This is why sambas often are a bit delayed. The poor stuff also takes 10 - 15 sec to hit the brain and switch it off.
I suspect that the reason that a forceful exhale may induce a shake is the effort that goes into the blast may rob the last critical bit of oxygen as well as and prolly more importantly, the pressurisation of the lungs and contraction of resultant rib cage muscles and diaphragm may exert a pressure on the heart and arteries carrying by now oxygen poor blood to the brain, slowing or even interrupting the oxygen depleted flow for a second, enough to cause the mainframe to blink and you lose your wallflower status .. The dance is on.
Dont worry about blast clearing your snorkel. Do it. If you are on the limit you will have forgotten to drop your weightbelt and will blast away when you hit the surface anyhow, as you will more than likely not be too cogniscant of your actions at that time. Surfacing and spitting out your snorkel to avoid clearing as a routine it is not an option.
Under those circumstances i would prefer you have the snorkel in your mouth because if you samba and black out you may just end up face down, blasted snorkel in and a clear air passage and auto recover. Without the snorkel you would be very lucky to end up on your back.
Some say that a SWB with snorkel in is bad cause the water flows into the mouth directly through the snorkel.
I dont think this is so because the mechanism that keeps the mouth closed also keeps the tongue jammed into the mouth of the snorkel and the soft palate and the tongue together, stalling any water ingress until this inevitably also releases at the time the closed mouth would have relaxed and opened anyhow.
Skin
