If the display is on, it will be turned off.
On some devices it takes about 3 seconds for it to really turn off, on others it might take around 10 seconds. Either way, a fully black screen will be shown while the screen doesn't really turn off.
The action attempts to set the display's timeout to the minimum possible value and displays a black screen until the screen turns off. After it runs, it resets the value to its original state.