The Perpetual Power Supply is a simple device.
The prankster plugs in the usb cable of the device into his device (or other device) and plugs the power adaptor into his device as well. The device then powers itself.
Obviously, it's really just a disguised battery pack that only charges if the
USB cable is plugged in. Probably would be a bit expensive to manufacturer, though presumably it could also be used as a portable battery pack.
Or you could just give the device two usb ports and claim it allows two devices to charge each other.
This is the kind of joke that relies on the presentation skills of the prankster as well as on careful selection of a sufficiently non-technical victim.