So, how does one actually use it? I get how to wire it, it’s the software control, that is unclear…
As Dan said, the OSPi software has a relay plugin that shows how to turn on/off relay.
The microcontroller-based OpenSprinkler has started supporting the relay from firmware 2.0.9 — it’s implemented by allowing a station to activate relay (similar to the way Master station is handled). Also, you can define a relay pulse value, which defines if the relay should be pulsed open/close, or should remain open as long as the associated station is open. Similar support will likely be integrated into the OSPi software in the future.