Why is rearward amplification a danger in multi-trailer combinations?

Rearward amplification (also called the 'crack-the-whip' effect) is the phenomenon where steering or lane changes at the tractor cause progressively larger movements at each subsequent trailer. A modest sway at the tractor can become a violent swing at the rear of a double or triple trailer, often enough to cause that trailer to overturn or jackknife. The rule of thumb is that doubles have around twice the rearward amplification of a single trailer, and triples around three times. The defense is steering smoothness: avoid sudden lane changes, do not whip the wheel to dodge debris, and increase following distance so you have time to slow gradually rather than swerve.