A Car 'fishtailing'. Fishtailing (also called "tank slapping") is a vehicle handling problem which occurs when the rear wheels lose traction, resulting in oversteer. This can be caused by low friction surfaces (sand, gravel, rain, snow, ice, etc.). Rear-drive vehicles with sufficient power can induce this loss of traction on any surface, which is called power-oversteer.
During fishtailing, the rear end of the car skids to one side, which must be offset by the driver counter-steering, which is turning the front wheels in the same direction as the skid, (e.g. left if the rear swings left) and reducing engine power. Over-correction will result in a skid in the opposite direction; hence the name. Without a proper driver's reaction, the fishtailing vehicle will spin completely.
Most modern rear wheel drive cars solve this problem by using a form of traction control which limits engine power when fishtailing occurs. The ability of the rear suspension to keep tires in contact with, and perpendicular to the road is also a key factor in the amount of grip available through the rear axle. For example a live beam axle suspension will have far less grip on a bumpy road than an independent rear suspension, due to its far greater unsprung weight, and forces from one wheel being transmitted through the axle to the other wheel, leading to the tire being out of contact with the road surface more of the time.
Similar behaviour is evident during heavy braking in all types of road vehicles, due to weight transfer to the front. This can be mitigated by re-proportioning the braking forces (more to the front, less to the rear) to keep the rear wheels from locking up. Most modern cars use anti-lock brakes (ABS) which addresses this problem. Older cars may have less sophisticated technical systems for lessening this tendency or the driver alone must actively modulate the brakes.
Fishtailing is also a term for the police pursuit technique of deliberately inducing rear instability in a suspect vehicle (see PIT maneuver).