JS Bin is different from other pastebins in that it saves by default automatically. This means you'll never lose your work. However, sometimes you want to throw open JS Bin and test some code without JS Bin ever knowing you were there. This is "sandbox" mode.
The URL is simply "slash-dash": http://jsbin.com/-
JS Bin runs as it normally does, but doesn't save. If you want to save, just use the keyboard shortcut ctrl+s