Please explain how two-way switch with Shelly 1L works

Hi,

I have seen multiple posts on the Internet regarding this picture for connecting a Shelly 1L with two switches:

Exactly what I need, and everybody says that this works, so who am I to disagree.

However, before I try this at home, I would like to understand the diagram and I fail to understand one point:

- Suppose the light is off, then their is no load on the line from Shelly 1L output O to the light.

- However, if there is no load on the blue line, how can Shelly 1L detect the position of he two switches on inputs SW1 or SW2 to turn the light on?

I would appreciated, if somebody could explain what I'm missing here.

May thanks,

Frank