You have to press the copper lever. This bends the disc and generates a voltage. But when the lever touches the silver surface, the piezo disk is short-circuited. A sharp switching edge is created, which should excite the quartz to natural oscillations. In a suitable receiver you should then hear a pling or plop.
In addition to the quartz and a capacitor with 22 pF, a diode is also built in. If the disc was discharged, it would charge in the opposite direction when the lever was released. The next time the lever is pressed, it would only just reach zero volts again. But this prevents the unwanted charging. Using the oscilloscope, I first examined the direction in which the disc would charge when actuated. In this case a positive voltage was generated. From this then the installation direction of the diode resulted. So you can press the lever as often as you like and generate more pulses. By the way, at that moment you hear a soft click of the piezo disk, because it suddenly relaxes a bit with the discharge.
As receiver I used the Elektor SDR-Shield. In CW mode I listened for a signal according to the crystal frequency at 4915 kHz. And indeed, a few Hz lower a signal can be received. It sounds like a plop. I can press the lever as often as I want and get quite reliably similar signals. But unfortunately the transmitting power is very modest. You have to get very close to the receiver input to receive anything. By the way, to the right of the frequency you can see signals that do not come from the piezo transmitter but from another source.