What is a jackknife and what most commonly causes it?

A jackknife occurs when the trailer pivots out of alignment with the tractor, folding to one side like a folding knife. The most common causes are trailer wheel lockup during hard braking (the trailer continues forward by momentum while the tractor decelerates, pushing the tractor sideways) and abrupt steering or braking on slippery surfaces. Once a jackknife begins, recovery is extremely difficult; prevention is the only reliable defense. Prevention measures include: ABS on tractor and trailer, smooth braking (especially on slick roads), avoiding sudden steering inputs, maintaining safe following distance to eliminate the need for panic braking, and proper load distribution. If the trailer begins to swing during braking, ease off the brake and let the trailer wheels regain traction.