What is a 'jackknife' (mise en portefeuille) in an articulated semi-trailer combination and what causes it?

1. The tractor drive wheels lock or lose lateral traction, causing the tractor and trailer to swing uncontrollably toward each other like a closing folding knife
2. The trailer kingpin breaks and the trailer falls flat onto the road surface
3. The engine stalls while climbing a hill and the truck rolls backwards
4. The steering wheel spins freely due to broken steering column splines