haha I need the “previously on…” clips at the beginning of MOST shows. I r dum.
After OpalCat & MrDibble had both said that it was open, then closed I thought I had been imagining it.
Ahh well a very minor nit pick on a show with 10 seasons worth of good viewing, i can live with.
I think they did explain it once in season 9 or 10, but I watched 3 seasons within close proximity so cant say which episode it was.
As others have said, the Iris works due to it’s proximity, it’s within something like 3 micrometers of the event horizon (the glowy liquid stuff), so nothing makes it through enough to become manifest, though it still hits with enough force to make a “Whump” sound (and in one episode, at the end of season 7, something hit the iris with enough force to make the room shake). Presumably this applies to the flush as well as anything else.
ETA: To add to the “proximity” argument, recall that the coverstone used on the Egypt stargate survived at least one attempt to gate through, as witnessed by the squashed Jaffa guards that General West and Colonel O’Neil look at in the special edition of the movie. Of course, in one episode in season 7 (“Death Knell”, I think), we see that the flush from an overturned stargate can dig a fair-sized hole in the ground, but that might be a function of how loosely packed the dirt is compared to stone.
Also possible that the Iris is just made of something that is strong enough to shrug off the flush. IIRC, the Ori ship that was destroyed was caught in the flush of a Supergate, wasn’t it? In any case, the nature of the gate it was next to was something significant, like the black hole they keep tricking troublesome people and societies into gating to. I haven’t seen the episode, and know of the incident only by rough description.
And yes, I also maintain that there has to be some poor guy who joined the Air Force to travel the world and do cool stuff, and finds himself wiping the back end of the iris with a squeegy and a bottle of Windex in a cave beneath a mountain in Colorado all day, before going home to drown his bitterness in a bag of popcorn and a DVD of Iron Eagle. Normally I’d assume it’s Sergeant Siler, the Air Force noncom on the show known for getting all the crap jobs (like testing out body armor with Teal’c), but I’d bet in this case it’s whoever works for Siler. 
Ah yes, the Dramatic Exit Detector.
Well, obviously the original gate builders were smarter than us. They’ve got a variety of safety features we (that is, Carter) don’t understand fully.
For instance, if you dial out from earth to a gate in space, the gate somehow knows it should hold the air back on our side.
At least once they’ve gated to and from a world where the other gate was submerged, and water didn’t gush through. Although in the particular episode I’m thinking of, it turned out the water itself was made up of sentient alien life forms who had the ability to resist passing involuntarily through the gate, and the gate seemed able to tell whether some of that water actually did intend to pass through.