I was told this is called a Master Cylinder and that it acts as sort of a automatic check valve for when the sewer system from the city backs up and it has a motor which cuts on to pump the water back the other way.
Is it a pump or is it a shutoff valve (controlled by motor triggered by flowswitch)? If there’s a backup from the sewer, I can’t see how the pump is going to force it down that pipe if there’s blockage. Or will it pump it down another egress?