The jbpm-human-task-services is emiting CDI events:
All the events are hidden from the regular user that is not interested in them but, because the Human Task WorkItems rely on these events to work you can find an example usage of them here:
https://github.com/droolsjbpm/jbpm/blob/master/jbpm-human-task/jbpm-human-task-workitems/src/main/java/org/jbpm/task/wih/ExternalTaskEventListener.java
That's the listener which is Observing events that are being emitted by the HT services.
All the tests inside the jbpm-human-task-workItems are using these events to notify the session about a task completion/abortion ->
https://github.com/droolsjbpm/jbpm/blob/master/jbpm-human-task/jbpm-human-task-workitems/src/test/java/org/jbpm/task/wih/HTWorkItemHandlerCDITest.java
If you debug those tests you will see where the events are being emitted and observed :)
Cheers