How do GE Add On switches work?

So I have a couple switches working in my house, one of which is a functioning 4-way switch. It is basically configured like the picture below.

Question: How do the add-on switches function? Do they send a signal to the main switch or what?

The GE add on switches communicate to the GE master via pulses over the physical traveler wires. They don’t have a radio inside and the add-ons will be invisible to SmartThings.