It does have a back-up valve, but for some reason becuase of the direction of surface water flow, the back-up valve floats across the steps so that when it turns on it actually pulls the polaris into the step.
Hard to describe via text without an image, but imagine the polaris stuck on bottom of pool at the bottom step. To its right is the wall. The back-up valve is along the surface, such that when it goes on, it pulls the polaris into the step. On occasion (by rare chance) the valve is pointing away from the steps and it is pulled away, but this is not the norm. I wonder if there is a way to manipulate the backup valve (?shorten the length) to move it closer to the unit to allow it to pull away?) so it is not floating near the surface?