What is an anti-portal?

This isn't a portal story, I think, but the
cover just screams "portal" to me!

I love stories about portals to other worlds, either fantasy worlds, like Narnia, or gates back in time like the Anubis Gates, or science fiction portal like Stargate; even gates that guard mysterious secret places, like the Secret Garden.

Some recent wonderful portal books I've read are The Archived, by Victoria Schwab, Daughter of Smoke and Bone, by Laini Taylor, the Iron Fey series by Julie Kagawa, Incarceron by Catherine Fisher, Wildwood Dancing by Juliet Marillier, and the Wings series by Aprilynne Pike.

I must not be alone, because Goodreads even has a list of books that include Doors, Portals and Gates (discovered some of my favorites on this list, and a bunch more I want to read).

As I writer, I'd love to write a portal story. But of course, it would need an original twist.

The first way to put a fresh spin on an old but good idea (like portals) is to think of its exact opposite. Such as an "anti-portal."

But what, exactly, is an anti-portal? I've been thinking about  this  for a while now, and I haven't had any light-bulb moments. But sometimes when I'm reading other stories, I get a hint of what I'm looking for in my imagination:

Here's a portal excerpt from Wildlwood Dancing that has a "different" sort of portal:

At full moon, the walk to Taul Ielelor was far shorter. At full moon, everything was different, everything was upside down and back to front. Doors opened that were closed on other days, and those whom the human world feared became friends. The Bright Between was a gateway: not a threat but a promise. 

And this description, from Fablehaven by Brandon Mull, is neat, too:

The air trembled. On hot days, Kendra had seen the air shimmer in the distance. This was similar, but right in front of her. The ground seemed to be tipping. Kendra extended her arms and swayed as the ground teetered even more. There was a burst of darkness, an anti-flash, and Kendra stumbled.

So there's the "anti-" idea going on there.

When I saw the cover of The Burning Sky recently, with a dark stormy sky on fire (with a neat sort of phoenix or dragonish shape) that made me think of an anti-portal, too:

But aside from the visuals, what would an anti-portal actually do? If a portal takes you somewhere, what would be the opposite of that? That's where my brain has completely stalled. It's like trying to understand (without mathematics) what exactly dark matter and anti-matter is... we know that most of the universe is made up of it, but... it's really hard to wrap my brain around it.

So I thought I'd throw it out to the world. When your imagination hears "anti-portal" - what does it come up with?