This is intended to reduce the roll period of the hull by acting in opposition to the free surface effect.
This type of tank was first investigated by William Froude, but did not receive much attention until the 1950s when it was revived and used in many naval vessels.
They have the added advantage that it is possible to vary tank natural frequency by changes in water level and thus accommodate changes in a ship's metacentric height.
More recently, a variation of these tanks has been used in oil drilling rig applications where forward motion is of little relevance.
This concept utilises an axial flow pump to force the water from one side of the ship to other rather than allowing it to slosh as in passive systems.
The main disadvantage to this is that when the pump is operated there is a time lag for a sizeable amount of fluid to arrive at a tank, thus limiting instant roll stabilization.
The main advantages to this are operating cost reduction by replacing part of fuel consumption and control of water flow without any mechanical mobile device.