The article says: "The device he came up with consisted of an antenna and a cylinder with a thin membrane that acted as a microphone. Soviet agents stationed a

The article says: "The device he came up with consisted of an antenna and a cylinder with a thin membrane that acted as a microphone. Soviet agents stationed across the street from Spaso House would turn the device “on” by focusing a radio signal on it, which then bounced back to their radio receiver. When the ambassador or anyone else in the study spoke, the sound waves caused the membrane to resonate and alter the signal that returned to the Soviets, allowing them to hear the conversation." The wikipedia link in a sibling comment has even more information.