I was digging around in my garage today and came across a box that some of the pool parts had come in. Peering into it, I found a rectangle piece of plastic that is roughly the size of the skimmer opening. The plastic says "Hayward" on it and looks like it snaps right into the skimmer and maybe acts as some sort of flap over the opening. So, what is this flap for? When should I be using it? Should I even be using it at all? Maybe its purpose should be obvious to me, but I am afraid it isn't. Can anyone educate me on this?