This method causes the target Clock to behave as though its Timeline object's BeginTime were set to the time of the next tick. The timeline's actual BeginTime setting is ignored (but not changed).
This method changes the target clock's CurrentState to Active. The Begin method does not resume a paused clock.
This method has no effect on the timing tree until the next time a tick is processed. As a side-effect, the appropriate events will also not be raised until then. Calling this method triggers the CurrentStateInvalidated and CurrentGlobalSpeedInvalidated events.