Opposite lock is a colloquial term used to mean the deliberate use of oversteer to turn a vehicle rapidly without losing momentum. It is typified by the classic rallying style of rear-wheel drive cars, where a car appears to travel around a tight bend sideways. This technique can also be used for front wheel drive vehicles.
The term "opposite lock" refers to the position of the steering wheel during the manoeuvre, which is turned in the opposite direction to that of the bend.
The technique works best on loose surfaces where the friction between the tyres and the road is not too high, but can also be used on asphalt or other surfaces with high friction if the vehicle has enough power to maintain speed. In this case a light tyre pressure increase is recommended to avoid problems during or after maneuvers.
Before entry to the bend, the car is turned towards the bend slightly, but quickly, so as to cause a rotating motion that induces the rear of the car to slide outwards. Power is applied which applies further sideways movement. At the same time, opposite lock steering is applied to keep the car on the desired course. As the car reaches the bend it will have already turned through most of the needed angle, travelling sideways and losing some speed as a result. A smooth application of power at this point will accelerate the car into the bend and then through it, gradually removing the sideways component of travel. Overall, the bend will have been negotiated much faster than driving through it in a normal manner. In skilled hands, the result is a dramatic and fluid motion which looks incredibly natural. When executed poorly, the result is often the complete opposite.
For front-wheel drive vehicles, there is much less natural tendency for the rear wheels to break traction because they are not transmitting power, so often such vehicles are set up with a strong bias to the rear brakes, allowing the driver to control traction using the brake pedal. The brake bias (front/rear) may be continually controllable by the driver using a hand lever. Left-foot braking is the favoured technique for using opposite lock in a front drive vehicle.
A related technique is the handbrake turn, in which the rear wheels are deliberately locked in order to break the friction between the tyres and the road, allowing the car to be spun around a very tight bend or junction, etc.