Taco "power head"

We heat geothermally (energy extracted from well water.)
Much of the flow in the system is controlled with Taco Power Heads (or at least that's what these gadgets are called by the folks who install them...)
These are small electrically actuated plungers that open and close the valves on which they are seated.
I know that they have beeswax inside them, but have no idea how they function.
Can anyone out there give me a clue...?
Sincere thanks,

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