Wikipedia on Godwin's Law

Godwin has stated that he introduced Godwin's law in 1990 as an experiment in memetics.

Godwin's law does not claim to articulate a fallacy; it is instead framed as a memetic tool to reduce the incidence of inappropriate hyperbolic comparisons. "Although deliberately framed as if it were a law of nature or of mathematics, its purpose has always been rhetorical and pedagogical: I wanted folks who glibly compared someone else to Hitler or to Nazis to think a bit harder about the Holocaust," Godwin has written.

From the Usenet FAQs on Godwin's Law

As a Usenet discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one.

And somewhere in there:

Quirk's Exception: Intentional invocation of this so-called "Nazi Clause" is ineffectual.

My understanding is it's meant as an observation of what typically happens, you can't deliberately try to to end a discussion by invoking Godwin's law, and it doesn't count when the actual subject of the thread involves Hitler and the Nazis.