"The former," he said, acknowledging the compliment with a nod. "One of my inventions, in fact. The principles aren't too dissimilar from those behind the Breach, as I understand it - although obviously not on the same scale, in this case."

In this case. I mean, he totally could make a portal to another dimension and big enough for Slattern to pass through, if he wanted. And also if the Porter wasn't a thing. But it's an example he knows she'll get, anyway.