This is one of those devices which makes you wonder why more people didn't think of it. It's so simple, yet useful. One common problem which ends up afflicting most Amiga users at some point or another is blown CIA's. These are responsible for helping control various things in the Amiga from timing to the mouse and parallel port. The Amiga's parallel port and serial port actually have live pins which supply +5V. The A1000's serial port also supplies +12V. A common mistake is to plug serial or parallel devices into the Amiga whilst it is switched on which if you're not extremely careful and accidentally short the wrong pins will send power down the wrong lines and blow you're CIAs. This device is designed to protect the parallel port and prevent you from blowing you're CIA's. Presumably it achieves this by isolating the live pin. This might mean however that some devices which draw power from the parallel port (such as sound samplers) may not work with it attached.
Thanks to Michael Czajka