The phenomenon known as jackknifing happens when a tractor-trailer reduces its speed. The tractor slows down, but the trailer keeps moving and swings around beside the trailer, moving sideways down the road. The momentum can even cause both to tip over, but even if they don't, it still poses a huge risk to the truck driver, other motorists, pedestrians... pretty much anyone in the vicinity. The risk is raised that much higher, you can imagine, when the truck is transporting any sort of hazardous or flammable substance. Anyone who's ever watched an action movie produced by Michael Bay or starring Arnold Schwarzenegger could tell you that much.
To offset the danger, the Swedish truck manufacturer – which is no longer associated with the automaker despite a shared name and logo – developed a system called Stretch Brake. The system works by pulsating the brakes on the tractor in order to keep it under control under adverse conditions. The result, as you can see from the video above, is an entirely safer kind of truck – one which makes us wonder why no one had developed this kind of system before. That's Volvo for you.