Spindle lock.

On drills with a keyless chuck it's a type of gear[ing] that only works when transmitting power in one direction. I guess it's a bit like a ratchet except that it works on both directions. There's a specific name for it which I'm afraid I can't remember. :-)

Not a very good explanation, using 'direction' with two different meanings!

It's a type of gear train that will transmit power from A to B but locks when transmitting power from B to A.

At the very simplest level (but it isn't this) think of a worm gear which will transmit power from the rotating worm gear to the pinion but won't allow the pinion to be rotated.

A sprag clutch?

I presume like a Quaife differential? The only spindle locks I've seen are manual, where a peg stops rotation of a shaft.

That I could understand, but the motor reverses? A one way clutch could work if it were a gearbox reverse after it. Just curious. ;-)

Most decent drills have them to allow one handed keyless chucks. In most cases they just work, although sometimes you might notice them "catch" as you slow down when spinning something big like a mixing paddle or hole saw.