What does the term 'jackknife' refer to?

A jackknife occurs when the trailer swings out of alignment with the tractor, folding to one side like a folding knife (hence the name). It typically happens when the trailer's wheels lose traction during hard braking or when descending a slick grade; the trailer continues forward by momentum while the tractor slows or steers, and the trailer pushes the rear of the tractor sideways. Once a jackknife begins it is very difficult to recover from. Prevention is the key: anti-lock brakes on tractor and trailer, smooth braking on slick surfaces, avoiding sudden steering, and keeping enough following distance to avoid panic braking. If you feel the trailer beginning to swing during braking, ease off the brake to let the trailer wheels regain traction and the trailer straighten out.