Here is the ping process zoomed out to show the startup of the receiver and the beginning of the power transfer. The first two pulses on the upper trace (which is the TX coil voltage) show a high amplitude indicating there is nothing on the pad. Some time after the second pulse, a receiver device is placed on the pad. During the next ping, because the TX coil now has a load on it, a low amplitude is observed.  After the analog ping detects the presence of something on the pad, the TX will send out the digital ping (longer pulses with enough energy to wake up the RX side controller). At this point, the RX device sends back the modulation pulses on the COMM line. After several of these, the TX power starts to increase until the RX side regulator can start up and provide a stable output to the load.