Terminate X11 Application with Mouse

There is a feature in X11 that temporarily transforms your mouse pointer into a *"kill X11 application"* icon (I don't even know what to call it) and allows you to forcibly terminate a process that owns an X11 window. On occasion I've activated this feature accidentally and want to be able to do this consistently. What is the keyboard, mouse shortcut, or other means of activating this feature? How does it work? is it a legacy feature and/or only works under some environments?