Think of a guy lazily leaning against a hallway door to the street, holding it closed (this analogy isn't perfect, but you'll like it anyway). He's doing that because there's a broken elevator at the end of the hall. If the repairmen allow the elevator door to open at the end of the hall, the wind blowing down the hallway and down the elevator shaft will catch the edge of the door and pull it open, and all the office paperwork will blow out the door into the street.
So, along comes this pretty secretary (yes, this is a sexist analogy) in a very short dress, and the guy smiles, half turns and reaches out his hand to say, "Hi!" The beautiful girl gives his hand a little caress on the way by, and he turns toward her (away from the door) trying to follow her down the hall. You got it, the broken elevator opens at the same time, the door he was holding blows open, and paper starts flying out the doorway.
That's how a Jetstream works.
So, seriously, the large surface area of the bell shaped valve insert lightly presses against the holes in the valve housing, and no air goes into your mouth.
Meanwhile, there's this TEENSY, WEENSY, TINY upstream valve at the end of the tube, with a lever resting against the diaphragm. It takes almost nothing to tilt the lever and open that tilt valve. That's why the reg works so well. The opening of the TINY upstream servo causes a drop in pressure in the tube it's connected to, and the airflow down that same tube causes the pressure in the bell of the valve insert to fall, at which point the bell collapses AWAY from the holes in the housing (remember the door catching in the hallway wind and blowing open?) and you have BIG airflow out the myriad holes in the valve housing and into your lungs.
When you want to stop inhaling, you allow the diaphragm to rise up to its resting position (you're not sucking on the mouthpiece any more). When the diaphragm rises, the tilt valve goes back to vertical (closed), and the pressure rises in the long tube going to the diaphragm end. When the pressure in the inner metal tube rises, it blows up the bell of the valve insert, and that area is more than sufficient to close off the holes in the housing that were supplying 99% of your breath.
<1% comes from the little bubbles in the servo tilt valve, and 99+% comes when the servo mechanism pneumatically magnifies the effort and opens the valve housing openings. It's like a lonnng lever. Just a little effort triggers something else. In this case it's a PNEUMATIC (as opposed to lever-action) opening, that uses the air pressure of the IP to both deflate the bell of the valve housing and supply your breath. But close that servo valve and the valve insert blows up like a balloon and blocks the holes you were getting your breath from.
As for an HP o-ring failure or other nonsense, if the IP rises to the burst pressure of your low pressure hose, you'll lose all your gas pronto. So Poseidon puts a pop-off valve in the hose to protect it. If an HP o-ring fails the IP will rise to the 200+ psi range and the popoff will start bubbling. You'd better start a Controlled Emergency Ascent right now.
But here's the "upstream valve" fallacy that plagues Poseidon:
A true upstream valve regulator (not a Jetstream) has the entire valve shut TIGHT with a high IP or upstream valve failure. You ain't getting no air no how. But the Poseidon upstream valve is just a tiny SERVO valve that controls the pressure in the soft bell of the valve insert. The ONLY way it can fail shut is if the lever at the top of the gauge breaks off from technician mishandling and cracking. Just because hose pressure has risen from a 1st stage problem, you can still breathe the reg. When you suck in on the mouthpiece, the diaphragm still moves, and increased IP is not nearly enough to overcome the mechanical advantage of the diaphragm pressing on the lever.
You still have a first stage failure and your air is burping out the hose popoff (or the valve body safety in the newer 3960). Your air is disappearing both ways. But at least you can still breathe the reg on your way up because your LP hose didn't burst. The Jetstream is NOT an upstream regulator. It's a pneumatic regulator with a small upstream component that does not fail the device.
And if the servo lever really does break off? You still have your octo. That's akin to a diaphragm failure in a standard second stage.
Okay, so the pretty girl was really just the rush of air down the hallway pulling you away from the door. But wasn't it nicer with her there?
