It's an ingenious application of the Fourier transform: make a recording of someone reciting text, convert the time-domain recording into a frequency-domain spectrum, then apply the resulting spectral lines to individual piano strings by way of mechanical actuators.
This would be a great movie special effect ... a femme fatale is tiptoeing through a dark room, when a nearby piano suddenly speaks up, saying "You aren't safe here", in a spooky voice that is very clearly a normal piano being forced to utter speech sounds.
