Liquid ring pumps work according to the positive displacement principle.
The impeller placed eccentrically in the housing forms the central element.
If the housing is partially filled with water, a liquid ring that will rotate along will form once the revolutions are fast enough.
The placement of the impeller together with the resulting liquid ring creates cells that are separated from each other.
These cells enlarge (suction area) and reduce (compression area) continually with every turn.