What is half-speed mastering and why should vinyl lovers care?

In the pre-CD era, there were all kinds of attempts to make vinyl sound as good as it could. That included things like direct-to-direct recording (time consuming and technically limiting) and something called “half-speed mastering.”

Direct-to-direct has all but disappeared but half-speed mastering has made a comeback with the vinyl resurrection. I’ve bought a few such records over the last few years and (to my ears), these versions do sound a bit better,

But what is half-speed mastering all about? This explains everything. (Via Gord)