It is reliably reported that Houdini is open again at Great Adventure.

I never really got what that ride is. Is it a walk through, a motion simulator, or a dark ride? And why was it closed to start with?

It was closed for the same reason(s) that other things like Twisted Twins and Mile High Falls at Kentucky Kingdom were closed.

The ride is a form of Haunted Swing, and it is best not to say much more than that. Riders perceive that the entire room swings about them during the...uh...performance. It is not a walk through, dark ride or modern motion simulator of any kind. Ride seating is in what could easily be church pews.

I've never understood how these attractions operated... obviously the riders aren't completely flipping upside-down, but does their seating move at all? Or is it just the whole room completely flipping around the guests?

Yes, this is a Terpy answer, but it fits. The answer is yes. Actually, it really is. Yes.

...

So the riders don't flip fully upside-down...

But they do move...

And the room flips around the guest...

?

Again, yes.

Terpy, there's an echo in here...

I assume you actually want the answer or you wouldn't ask, so I won't riddle you. The gondola does swing, but to about fifteen degrees in each direction. The room itself is a giant barrel, pretty much, that rolls around you in increasing, synchronized arcs that mirror the swing of the gondola. They're actually pretty cool if you ask me, especially the big budget ones with synchronized music and special effects. And contrary to what you may think or how silly the videos may look, they're very very disorienting. I've heard a lot of people just getting off the ride or months later just adamant that the ride really does go upside down.

There's a POV video of one of the more famous, Hex at Alton Towers here. As well as a behind the scenes video of the spinning barrel

.

And of Great Adventure's here:

Sweet! I thought these were only in Europe.

This would be a nice Crypt replacement...

Carowinds had an Arrow made one from 1985 to 1999, and at one point, Hersheypark had one.

Also another great overview of the ride is at Great Adventure History (greatadventurehistory.com)