Suppose you accidentally click an application’s Dock icon so that it launches (or you otherwise unintentionally launch an application.) You may want to cancel the launch immediately rather than wait for it to finish and then select quit -especially if that program takes a long time to open.
You can cancel the launch by force-quitting from the Dock. Just click and hold on the bouncing icon for the launching application. The command that normally reads Quit will now read Force Quit. Select it, and the launch will be instantly terminated.