This is the async-ready equivalent of AutoResetEvent, similar to Stephen Toub's AsyncAutoResetEvent.
Like other "events", an AsyncAutoResetEvent is either set or unset at any time. An AsyncAutoResetEvent can be changed from unset to set by calling its Set method. When a WaitAsync operation completes, the AsyncAutoResetEvent is automatically changed back to the unset state.
Moving an AsyncAutoResetEvent to the set state can only satisfy a single waiter. If there are multiple waiters when Set is called, only one will be released. (If this is not the behavior you want, use AsyncManualResetEvent instead).
When an AsyncAutoResetEvent is in the set state (with no waiters), Set is a noop. The AsyncAutoResetEvent will not remember how many times Set is called; those extra signals are "lost". (If this is not the behavior you want, use AsyncSemaphore instead).
The task returned from WaitAsync will enter the Completed state when the wait is satisfied and the AsyncAutoResetEvent has been automatically reset. That same task will enter the Canceled state if the CancellationToken is signaled before the wait is satisfied; in that case, the AsyncAutoResetEvent has not been automatically reset.
You can call WaitAsync with an already-cancelled CancellationToken to attempt to acquire the AsyncAutoResetEvent immediately without actually entering the wait queue.