A light switch is the simplest of electrical devices. Flip the switch, electricity flows. Flip it again, electricity stops.

In the diagram on the link they are using a three conductor cable after the first three way switch. Black ... They shared the white neutral wire. It sounds like you got it figured out, but ...

Eaton places screw terminals on three of the four sides of the device, making wiring a bit complex ... and they also illuminate when the switch is having trouble connecting to the network.