When it gets cold enough heat pumps stop working so there has to be another source of heat. Rumor has it that this is the inverter. I can't tell you how it works because I don't know but I can point out a couple of things from which we can deduce how it probably works. Refer to the following sketch:
The items A, a, B, b, C and c are solid state switches which can be "gated" on and off electronically. When driving A, b and c would be turned allowing current from the battery to flow into the coil at 10 o'clock and out of the coil ar 2 and 6 o'clock. This results in a magnetic field directed at 10 o'clock. Those switches are then gated off and B, a and c are turned on. This results in current flowing into the coil at 2 o'clock and out of the coils at 6 and 10 resulting in a field oriented towards 2 o'clock. Finally those are turned off and C a, and b turned on giving a field oriented towards 6 o'clock. The sequence then repeats with the result that one has a magnetic field rotating in a clockwise direction in steps of 120 °. In fact there are more than just 3 coils and the field rotates in smaller steps.
The resistive power dissipated by a switch (or any other device) is the product of the voltage across the switch with the current through it. When any of the switches are turned off the voltage across it is high and only a small leakage current flows through them so that the power dissipated is small. When one is turned on there is a large current through it but the voltage across it small and, again, the power dissipated in it is small. But the switch has to get from the off state to the on state and in doing so it passes through states where both current and voltage are intermediate and power is dissipated. Engineers strive hard to get the switch from off to on and back to off as fast as possible to minimize the energy (integral of voltage times current with respect to time) lost to heat. So one strategy for getting heat from the inverter, the one that would be used when moving and more battery heat is required than supplied by the battery's internal resistance, would be to slow the switching speed (apply a gating waveform with a more gradual rise time).
When trying to pre-warm the battery (car not moving) a possible strategy is to turn all the transistors on part way. No current flows through the motor coils under these conditions - just through the transistors which will get warm. Yet another strategy might be to turn on one capital letter transistor and the other two small letter transistors. Current then flows through the transistors (which get warm) and through the coils which also get warm (the motor is also on a glycol loop). All the energy going to the motor gets converted to heat in this arrangement as the motor is not turning and none gets converted to mechanical energy.
Thus my guess is that one or the other or some combinations of these techniques is used for supplemental heat in the Y and will be in the CT when it's too cold for the heat pump.
I^2R is I^2R whether the current is flowing through a resistor or transistor so this method is just as efficient as a resistive heater. What other form could the battery energy be converted to?