How does a reservoir control circuit work?

A reservoir circuit is meant to keep a tank full. This can be achieved by using normally closed float switches. It is important to note that a reservoir circuit, much like a sump circuit, is a low voltage release circuit. This is important to remember in light of safety issues, as a motor could start automatically upon the return of power.

[poorboybmx]

What operates the holding contacts? the diagram only shows two float switches