This is a really unique, smart accessory. The Power Plug will turn any controller into a "super controller"! The idea is to give a controller, regardless of type, basic functions like turbo fire and slow motion, as well as other functions not commonly available on most controllers. There were versions of the Power Plug for both the Super Nintendo and Sega Genesis.

You plugged the device into the SNES deck, either port 1 or 2, and connect the controller to it. The device is a box, about 4" x 3", and about 1" deep. On top there are six buttons; Auto fire, Steer, Thrash, Pro, Slow/Turbo and reset. Above the first four buttons are indicator lights, and the slow/turbo button is one long toggle button. The reset button, I assume, resets all the settings to off.

The Power Plug allows you to turn any controller into a turbo fire joystick, with auto fire option, as well as slow motion. Thrash mode lets you execute preprogrammed moves for games like Street Fighter II. Pro mode lets you program in your own moves. Steer allows you to have "power steering", for a more accurate feel when playing driving games, like Super Mario Kart.

Source: EGM #53

Info and pics ripped from eBay