What is the 'jackknife' effect (efecto tijera) in articulated tractor-trailer vehicles?

1. The cutting of cargo straps by sharp steel corners
2. A loss of directional stability where the tractor rotates violently relative to the semi-trailer around the fifth wheel coupling, folding into an angle like a pocketknife
3. The lowering of the front landing gear while driving
4. A puncture in both steering tires simultaneously