Kevin is essentially correct, but allow me to add some details. They are specifically called Wagner bypass valves. At the ends of each chamber are piston like valves. A large spring (or springs in the case of 2926) sits in between the two valves pushing them away from each other. In this position, the two sides of the piston are interconnected with the intention that neither compression nor vacuum would develop in the cylinders when drifting down grade without steam. Once the throttle was opened, pipes visible on the outside of the timing valves would route pressure from the valve chamber to the far ends of the Wagners, pushing against the piston like valves which would travel inward compressing the springs and closing the interconnect passage between the main piston faces.
My understanding is that the throttle would periodically be opened on long down grades to refresh the lubrication (though the Santa Fe power that used these had lubrication points directly to the cylinder and timing valves as well as at the steam inlet). Test data showed that some pressure and vacuum would still develop in the cylinder, so it is likely that the periodic burp of steam may have helped relieve the situation...they may have retrofitted stronger springs after the tests I read about to improve their performance, but even so there was demonstrable savings of fuel and water. I have talked to at least one old timer who said they still tended to drift with a light throttle...I know at least one of our engineers has been studying up with the old timers on how best to use them, but am not sure if most of our guys drift with an open throttle or not...
Probably more than you wanted to know, but there you go...
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