Ethernet transformer 3-line common mode choke

Attached here with is a diagram of ethernet transformer. I couldn't understand how the 3-line common mode choke works? (on cable side)

It appears to me that this common mode choke isn't doing anything?

Here is my understanding:

1. Three wires (cable side p/n signal + ground) are wired together on the same coil.
This ensures the total current is almost zero.
2. For common mode signal from cable side, it appears the choke isn't going to anything. The 2x current in ground wire will cancel all flux from signal wires and choke appears low impedance for common mode currents.

Any suggestion to help understanding?

Thanks,
Neo