How does circuit network wildcard work (the spring shape wild card)

[Z0MBE]

I haven't seen anything online that describes it's purpose, the item description seems very vague to me.

[Fel]

It is (mainly?) for train interrupts.
When you connect a circuit network to your train stop, you have the option of sending the signal to trains that are at the stop.
That way, the train can use the signals as part of its departure conditions as well as interrupts (while at that station).

Like the other wildcards, the circuit network wildcard changes to the symbol of the signal that matches the interrupt conditions and can be used to determine the destination.

If you want an example, you can send a signal for iron ores to a train at the stop and have it go load up on iron ores (and possibly unload said ores as well), without requiring to have iron ores in the waggons yet (which is how the item wildcard works for example).