Could we imagine a kind of venturi effect?
This isn't a venturi effect - it is a centrifugal effect. It is similar to a cyclone air filter.
What is does mean is that there is a region in the centre where the air pressure is lower than at the periphery. Or rather, the periphery is at a higher pressure than the centre. The centre will still be around atmospheric pressure - or there would be no flow of air out of the device.
Despite what it says on the wikipedia page, there are still ongoing arguments about the distribution of heat within the swirling air of a RHVT.
However, the increased pressure around the periphery will help to keep the plasma in the centre of the swirling gas - as previously shown on the ENG8 website.
As Alan Smith mentioned, a while back, it will be interesting to know how much NOx and Ozone come out of this thing.