What does "jackknife" mean in relation to tractor-trailer accidents?

In the context of tractor-trailer accidents, "jackknife" refers to a situation where the trailer swings out and creates an angle with the cab, which can often lead to a loss of control. This typically happens when the trailer's wheels lock up during a sudden stop or when braking occurs on a slippery surface. The disconnect between the cab and the trailer's movements can result in the trailer folding in toward the cab, resembling the folding of a jackknife. This can create a very dangerous scenario, as it limits the driver's ability to steer and maintain control of the vehicle. Understanding this concept is vital for truck drivers, as it emphasizes the need for careful braking and speed management, especially in adverse conditions.