Anonymous has described it pretty well - it's a turn achieved by using the handbrake rather than the steering wheel. In my case, the drive test instructor said 'turn left' but I was suffering a bit from the flu and the instruction didn't really penetrate my brain too well and by the time I turned I was going a bit too fast, so he pulled on the handbrake and the car went 'squeeeeeeeee' and slid sideways up the road, onto the pavement, stopping a few inches from a parked car. At that point, he decided it might be safer if he drove us back to the test centre. I didn't argue. I was too busy trying not to faint from a temperature that was nearing dangerous. In fact, that particular bout of flu nearly put me in hospital so I probably shouldn't have taken my test at all, but I had paid for it by then so I figured that it would at least be practice.