Perhaps the most remarkable thing about this engine is how conventional it looks. It is designed to work as a wet expander- in other words liquid helium forms actually inside the engine. Claude had dreadful troubles with lubrication when was liquefying air inside an engine; the fact that it can be done routinely with helium presumably demonstrates what progress has been made in dry lubrication since 1900.
Its operation is pretty much self-explanatory, but note that the valves are at the bottom, presumably so the liquefied helium is easily cleared through the exhaust valve.
Piston expanders like this one are commonly used in the temperature range 20 - 40 K. (Kelvin or degrees Absolute) Wet expanders are preferred as they are more efficient than dry expanders, in which the gas cools but does not liquefy. Typically wet piston expanders are 80% efficient compared with dry piston expanders at 70 - 75%.
It is well known that in steam power generation turbines are more efficient and more reliable than reciprocating engines. In the helium business things do not work the same way. Turbines can be and are used as expanders, but piston expanders are in general more efficient, and maintain their efficiency over a wide range of flow rates, making plant operation easier.
The power output is tiny compared with that required to compress the helium in the first place, and is usually not worth trying to exploit it. However the machine must be loaded to prevent it overspeeding, usually by driving a fan or water brake.
From Cryogenic Engineering Ed B A Hands. Academic Press 1986