It tests whether your mouse movement looks human–we’re really bad at things like moving in straight lines, so it’s pretty evident from a mouse movement log whether you’re a human or a simple bot. It also takes a bunch of auxiliary browser/environment data into account. It’s not perfect, but it’s complicated enough to defeat to provide fine protection against cheap spam.
Per the article:
It looks like “threatens a lawsuit” is being used here because “sues” would be inaccurate (since the suit has not been filed).