Nothing is truly "failsafe". When the first stage pukes and the air "hits" a normal downstream second stage, the second stage goes into a freeflow. The downstream valve is the first stage's "safety valve" and you can still breathe until your air runs out, which it will do rather quickly.
On a Poseidon, when the first stage pukes the safety release on the first stage does the freeflowing. If it didn't then you would NOT be able to breathe on your second. SOOOOOOO, if you put your Poseidon on a regulator that does NOT have a safety valve, then NO you would not be able to breathe on the thing when the first stage pukes, at least until the hose explodes!
Intermediate pressures are completely adjustable on all of the Poseidon first stages I have seen... I would feel comfortable using them with any second stage out there. I WOULD not, however, put only a Poseidon Second stage on a non Poseidon first stage, UNLESS I equipped the first stage with a pressure release valve (avilable at caving shops for their argon bottle set-ups) OR I had another second stage attached (as an octo) that WAS a downstream design.
One last point... MOST freeflows are caused by the second stage and NOT the first stage. This type of freeflow is NOT what we are talking about here, and they are usually stopped by putting your hand over the mouth piece and occluding it. We are talking about when the first stage regulating mechanism goes awry and delivers too much pressure to the second stage. Putting your hand over your mouthpiece will do squat for this type of freeflow since it comes from a mechanical problem. You should have your regs serviced immediately if you encounter this type of freeflow.
Does this answer the question???
