If you didn't realize, diesel engines do not have throttle plates (butterflies) like gasoline engines which controls the amount of air going into the system.
The way it is controlled is through the amount of fuel which is injected into the engine. The more fuel, the faster the engine will run. The way this is counteracted is through the use of a governor. The governor can be either mechanical or electronic in nature. Depending on the throttle position, the governor will allow more/less fuel into the system to speed-up/slow-down the RPMs. If something happens to the governor, or it isn't doing it's job, the engine can go into a runaway situation.
If a runaway occurs, the engine will continue to gain speed until it reaches a point where it cannot go faster and will stabilize at that speed. This speed is usually well beyond the redline of the engine. Due to being at this maximum speed, the engine will soon self destruct.
There are two ways you can stop a runaway once it has begun:
- Stop the intake of air
- Stop the fuel from entering the cylinder
The easiest of the two, in most cases, is to stop the air from going into the engine. If you saw a video of a runaway which they were able to stop the engine from self destructing, you probably saw them put something solid, like a piece of wood (I saw someone use a clipboard once) in front of the intake to shut off the flow of air. This will shut an engine down very quickly and prevent the runaway from doing any damage.
Runaways are far less common with today's electronic fuel injection, but can still happen. I saw a video of a brand new (at the time ... probably a 2012/2013 model) GMC pickup truck do a runaway where the engine went into thermonuclear meltdown. It got to a point and the engine started slowing down (melting pistons??), then the radiator blew out ... made a huge mess of things. Runaways have been occurring since the advent of the diesel engine. I don't expect to hear the end of them as long as diesel engines work as they do.