This thing turns the water flow on and off. At first I was a bit hesitant to leave something hooked up with the spigot turned on all the time, but I read through the instruction manual and it seems pretty failsafe. It will shut down when the battery gets low, etc. I have two lines hooked up to it - one going to the front of the house, and one to the back.
It's programmable, and quite versatile, allowing you to program three daily on/off cycles of variable duration, and you can select which days of the week it runs. I have it going for an hour and a half, four days a week, for now.