A jackknife happens when:
Correct. Jackknife is when the drive wheels skid and the trailer continues forward, causing the tractor to pivot — the rig folds at the fifth wheel like a closing knife.
CDL Practice Question #172
Category: Combination Vehicles
Why this question matters
This question is one of 55 in our Combination Vehicles bank. The reasoning above comes directly from the AAMVA CDL Manual, the source document used by every U.S. state DMV. Memorizing the underlying rule — not just the letter answer — is what separates passing on the first attempt from a return trip and another fee.