What does the term "jackknife" refer to in truck driving?

The term "jackknife" in truck driving specifically refers to a situation where the trailer swings out and folds up against the cab. This typically occurs when a truck loses traction, often in slippery conditions, or if the driver makes a sudden turn or hard braking maneuver. In this scenario, the trailer moves independently from the cab due to the pivot point of the fifth wheel, which can lead to a dangerous loss of control. Understanding this term is crucial for truck drivers, as recognizing the potential for a jackknife situation can help them adjust their driving techniques to avoid it, thereby enhancing safety on the road. The other options do not accurately capture the meaning of "jackknife" within the context of trucking and driving safety.