This illustration shows the basics of how the communication works: As soon as the secondary side receives any power (that is, from a digital “ping”), it wakes up and applies load pulses as shown. Extra load on the secondary will increase the primary side current. This effectively shows up as a sort of “amplitude modulation” of the primary coil voltage and current. The processor on the primary side can demodulate the 1’s and 0’s that represent the data based on measurement of the primary side current. The AC excitation (power transfer) runs at ~ 100 KHz (“carrier”) and data rate (“modulation”) ~ 2 KHz. (RX side load pulses can be done with a resistor or capacitor).