@anto - for old sprinkler controllers that don’t have a rain sensor port the rain sensor (which is normally closed) it wired in line on the common wire. When the rain sensor is wet (e.g. after a rain), the sensor opens and prevents the electrical circuit from being completed to the sprinkler valves. The sprinkler controller will still think it is watering and providing power out to the sprinkler valves, but as the common wire is open due to the rain sensor nothing will run. As your neighbor said, taking out the rain sensor and connecting the two wires together will take the rain sensor out of the equation.
