The intent here is to remove all unnecessary interactions with the middleware so that you can isolate the problem and debug a single service instance without interference.
The system invokes the onSessionLeave(…) method to unbind the current binding session when there is no more work for that session. Sending only one task ensures that the onSessionLeave(…) method is invoked shortly after the task completes.
Your update causes the session director to immediately start a session manager to service your application (preStartApplication="true").
Your application's session manager preloads a single service instance even before you send it any workload (numPreloadedServices="1"). The running service instance is not yet bound to a particular session.