Jackknife is a term that refers to a movement of a truck hauling a trailer where the trailer pushes the cab in a circle until it swings around and smacks the trailer. The vehicle looks like a jackknife folding if you looked at it from high in the air, hence the name.
There are many different reasons why jackknifing occurs, but the most common examples are bad weather and brake malfunction. Any situation where the trailer moves faster than the vehicle can cause a jackknife accident. This is extremely dangerous to the driver of the truck because the cab hits the side of the trailer with a lot of force.
The physics of the accident makes the cab want to fold underneath the trailer, which can cause a lot of damage. Jackknifing is also dangerous to other drivers in lighter vehicles, as in any situation where a large truck loses control.
A driver who jackknifes due to faulty brakes or some other malfunction may be able to get back on the road more quickly with commercial roadside assistance.