I hear the reflexed part, of reflex cans generally, takes very little pressure before uncorking as the gasses don't want to change direction so much.  But I hear the reflexed part is useful in reducing blowback as the front part of the can can vent into that part somewhat rather than just sending so much to the chamber.  

Is that the trick of the OSS cans?

The trick of OSS cans is the total distance traveled by the gasses inside the suppressor before they are expelled out the muzzle. Their reflex cans are unique in their design in that the gas travels from the muzzle to the back of the can, then forward, then back, then forward again out the front of the can. Add that to the travel of the gas being cyclical rather than linear (gas moves through chambers that rotate around the bore of the can rather than a straight passageway to the muzzle) and you can start to see why the design is effective in eliminating gas blowback.
Baffle cans take the gas exploding from the muzzle of a gun and decelerate it very rapidly with baffles, OSS cans gently circulate the gas through the inner workings of the can and it trickles out the front.

Yeah, makes a sense in theory, except I don't see how they are really getting all that action to occur without tapping the gas prior to the muzzle.   They can not make the gas do all these things until after the bullet has uncorked from the muzzle, so the gas has to do all these moves when under only partial pressure for like an inch or two in the muzzle device thing that the reflexed portion mounts on.    Well choked gas flows only at a couple thousand FPS or less, which at that point is slower than the bullet.  So, I am prone to call BS on their explanation of how the cans work as how could this choked gas flow be forced to go back and forth and back and forth all in so little time?

It is true that they have much less blowback than a non reflexed baffle can.  The reflexed portions of cans are shown to hardly pressurize when the bullet is still in the suppressor as supersonic gasses do not want to change direction easily, but that volume is useful in being a place for the depressuring gas to expand into as half of the gas rebounding from the baffles after the bullet has exited flows backwards.

Do you know how else you reduce back pressure?  By having a shorter can with less baffles in front of the muzzle.  I am not positive what is inside of of the forward section.  The guy on youtube says the "there is not a baffle in these cans." But elsewhere I have read that the forward part is more like a conventional can.  Well, maybe the front of their cans do not have anything in them which would make the front part of the can large single baffle on account of the end cap.

I would like to see a side by side test of their can with a simpler constructed reflexed can of the same length fore and aft and the same internal volume fore and aft, and the same number of baffles, if any, in the front portion.

I'd say away from OSS at this point. Their founder left the company with some pretty damning words about the board and took most of the brain trust with him.

http://soldiersystems.net/2016/02/22/russ-oliver-resigns/

So you're going to buy an OSS gen 5, AEM5, and Griffin SPR OTB and test for us?

yep, right after I buy a sound meter for how ever many tens of thougsands of dollars those cost.   Real data in controlled test is the only way, of course.  I know I am just speculating w/o resources to conduct said tests, but "extrordinary claims require extrodinary proof," has any one ever done any tests, maybe some old SS tests from back when they had a go w/ them?

So, apparently gasses would be lowing at 4000fps or better.  So, actually it is not so extroidanary to think that the gas might be ducted back and forth before the whole system depressurizes, still it would be nice to see side by side testing w controls.