Javadoc
Called when an event is about to be dispatched to the protocol (before it is dispatched).
The down handler thread will block until this method returns. This allows an implementor
to block indefinitely, and only process single events at a time, e.g. for single-stepping.
For example, upon clicking on a button "Step" in the Debugger GUI, the method would unblock
(waiting on a mutex, GUI thread notifies mutex).