Though I'm new to drifting and don't quite know how this fits in to making drifts easier, I would guess a "one way diff" is a locked out differential? In the monster truck world and with rock crawling you can put a spool in your diff in place of the spiders to lock the output shafts together to the diff, giving you 100% power to each wheel. This way when one wheel starts spinning during your drift, the other wheel will stay the same speed. Usually this is considered to make the car handle worse and more difficult to turn but the inverse may be true with drifting cars.

Now, anyone care to explain everything I missed in above paragraph?