I, uh, haven't yet. 😅

I'm not sure, what it's called in English. In German, we have the sexy term "Lendenwirbelsäulenblockade" for it.
The internet tells me that it might be a "lumbar vertebral subluxation" in English, although that Wikipedia article sounds a lot more like quackery than my German impression of it. It might also be a "lumbar blockage"...?

But basically, one of the many joints in the spine for some reason loses its ability to move. And often times, the solution is to apply a bit of force, to get it unstuck. You *should* talk to a professional about this, or at least a chiropractor, because force won't always be the correct solution.
But yeah, if you ignore those safety instructions, what you can do, is to slowly move your back into the position that the right cat is in (while on your knees and hands). Then slowly arch your back into the other direction. At some point, you might hear a pop, as the joint regains its ability to move and then that's that.