Poor Mojo's Newswire

Usefully, said the pair, the six wires inside a PS/2 cable are typically "close to each other and poorly shielded". This means that information travelling along the data wire, when a key is pressed, leaks onto the earth (ground in the US) wire in the same cable.

The earth wire, via the PC's power unit, ultimately connects to the plug in the power socket, and from there information leaks out onto the circuit supplying electricity to a room.
Even better, said the researchers, data travels along PS/2 cables one bit at a time and uses a clock speed far lower than any other PC component. Both these qualities make it easy to pick out voltage changes caused by key presses.
A digital oscilloscope was used to gather data about voltage changes on a power line and filters were used to remove those caused by anything other than the keyboard.

"The PS/2 signal square wave is preserved with good quality... and can be decoded back to the original keystroke information," wrote the pair in a paper describing their work.