I would imagine via a stealthy redirect so instead of Google->resource you have Google->resource->resource in your history.

I've also seen this behaviour (though I'll admit I put it down to shitty JS programmers not preserving history and not this, interesting).

But how does the Washington Post prevent the back button in the case above? You click the link which takes you to m.washingtonpost.com which in turn redirects you to www.washingtonpost.com but there's no back button available back to m.washingtonpost.com.