We had a bunch of family stay over the weekend and I was trying to explain to my bro inlaw how the pool plumbing works. He asked what happens when/if the skimmer gets completely clogged. I told him that it just automaticaly starts pulling water from the two main drains. So I plug one of the holes in the skimmer with a tennis ball and the pump runs dry . I thought for sure it would pull from the main drain and I could have sworn i've done it this way before . We tried several times with the same results. So fast forward to today, I put my diverter in the skimmer, hold the wier door closed and shazam the diverter float drops and the pump starts pulling from the main drains . So whats the difference between the diverter shutting off the water supply or another device or debris?