A semi-truck jackknife crash is what happens when the vehicle’s front cab and rear trailer are moving at different speeds. This out-of-sync motion causes the back trailer to push forward and sway out of the lane and off to the side.
This swaying motion is not like a subtle drift from one lane and back again like you might experience with a drowsy driver. Rather it resembles the action of a pocketknife when you fold it up. In other words, the trailer ends up being perpendicular (90 degrees) to the cab. Truck drivers may skid and completely lose control of the vehicle in this situation. The truck may even roll over before the vehicle comes to rest.