What is rearward amplification (the 'crack-the-whip' effect) in doubles and triples?

Rearward amplification is the way a quick movement at the front of the combination grows larger as it travels back through the connected trailers, like cracking a whip. A modest steering input or swerve at the tractor can cause the rearmost trailer to whip out with several times the lateral force, raising the risk of that trailer tipping or striking another vehicle. The effect is greater the more trailers there are, so triples are the most prone to it. Drivers prevent it by steering gently, avoiding abrupt lane changes and quick maneuvers, and slowing down — there is no way to recover a trailer that has already been whipped into a rollover.