A jackknife is when the trailer skids and swings out of line with the tractor, folding toward it at a sharp angle — the shape that gives the move its name. It usually happens on slippery roads or under hard braking when the trailer loses traction.
Jackknifes can cause serious crashes because the driver loses control of the combination. Safety systems like ABS help prevent the wheel lock-up that often triggers one, and careful braking on wet or icy roads is the first line of defense.