It's fairly obvious from the pictures. You find and trap a small, but good-bass-response speaker. Then you hook an 8-ohm to high-impedance baby transducer to it, and run that to the jack. Quality will depend on the qualities of the speaker, transformer, and the box you bolt it to.
A similar thing would have been made at one time by using a twin-T oscillator that was tuned to just not ...quite... oscillate so that any disturbance would make it ring. Low fundamental frequency gives a good bass thump. Suffers from not having proportional response to how hard you hit it, but mechanically much simpler.