System.Reactive.dll adds the AsyncLock class to System.Concurrency. It has a single instance method, Wait, which takes action.
Documentation page I found that she was just telling her the preliminary documentation, so I ask here: what does this class do?
It looks like some kind of thread-safe queue that performs actions in sequential order. I see that it is used in RX schedulers.
You might want to confirm the RX command.