It's the twisty thing that the cords go into in your sprinkler boxes. You can turn the sprinklers on manually with it. Just google the brand you have and there are probably at least 100 videos on it. Mine is a rainbird and you just had to unscrew 4 screws to replace and swap. Sometimes there is even just a rock or some crud blocking the seal so that it can't close all the way, so then it leaks out slowly and ends up kind of oozing out of the lowest sprinkler in the zone.