... or, at least, a mystery to me.
OK, new liner professionally installed, along with a new skimmer. The skimmer has two ports. Port #1 is connected to the suction side of the pump, as expected. However, Port #2 is attached to a line that just forms a "L" and re-enters the pool about a foot or so below the skimmer. This is described as an "equalizer line." I can't see that it does anything, except make it impossible to effectively use a vacuum plate with the vacuum.