A web search came up totally empty, not even finding this thread. Which only reinforces my recent impression that DuckDuckGo is suddenly no longer a good search engine. )-:
This is just a stab in the dark, but the word 'motion' made me think of something related to an earthquake or movement of the ground, like tectonic plates or fault lines where you can see how one side of stacked layers of the earth has moved relative to the other, but with the layers still intact, like a sandwich. So that might mean something like that the dance floor wasn't yet collapsing, even though it was shaking violently?
Again, just a guess.
Are we actually sure that the text isn't corrupt? I can't remember if this is one of the ones where in another passage we found words completely subsituted or misprinted. If so, the original could have been something completely different, like 'sideways,' 'sideward,' who knows.
Or could it be something that groups of people dancing do in clubs? Like, for example, a couple of lines or blocs of people, each pushing toward the other, sandwiching a third group in between them? That wouldn't be very safe ...