In my case, my main drain runs from the bottom of my pool to a second opening in my skimmer. My skimmer goes down below the current water level in my pool so as soon as I open the main drain, it starts filling the skimmer.

I suppose I could lower the water even further, pressurise the main drain, then raise the water level up a bit.

Can anyone tell me if this is necessary? I know the pool guy didn't do that.