I just finished this book, and I never understood why the phenomena/event was called "the spin."
It's an easy name for the idea that the outside universe is moving much faster and things would be seem to be spinning across the sky if they were visible.
From the book:
"Spin" was a dumb but inevitable name for what had been done to the Earth. That is, it was bad physics - nothing was actually spinning any harder or faster than it used to - but it was an apt metaphor. In reality the Earth was more static than it had ever been. But did it feel like it was spinning out of control? In every important sense, yes.