A plot twist is a small device similar in appearance to a plot device, but vastly different in effect.
Plot twists are nearly identical in appearance to plot devices, the only difference being that a plot twist appears to be made of solid gold, as opposed to the shiny, silvery metal of a plot device. A plot twist has the appearance of a cube, roughly the size of half a brick, made of solid gold. A large, red button is built into one side.
When a plot twist is activated, it vanishes in a puff of smoke. Almost immediately, an event will occur, usually a catastrophic one, and usually can have largely negative side effects on anyone, including the person who activated it. Events set into motion by a plot twist cannot be undone by a plot device.
"Plot twist" is a term used to denote a massive and unexpected shift in the plot of a story.