A variation in liquid level produces a change in the weight of the rod (partially submerged), which can be measured by the extension of the spring that supports the rod. The variation in the length of the spring is transmitted to the indicating needle via a magnetic coupling between a magnet on the end of the spring and a magnet fixed to the indicator.
The rod is always immersed in the liquid (not floating on the surface). This measuring principle is well suited to dirty environments.