The simplest way to see for yourself how boundary events work is to create a simple process with a human task and a timer boundary event set to a suitable time period, for example, one minute. Then, you can start an instance of the process and ignore the human task for a minute to see the boundary event fire.
Let's create this example now:
In JDeveloper, create a new BPM Application and call it BoundaryEvents.
Create new BPM Project called BoundaryEvents.
Create new Process called MyTimerEvent.
Move the End node aside to create some more space and add a User task and two Activity tasks to your process. Name the two Activity tasks as Normal Activity and Handle Timeout.
Mark both of the Activity tasks as draft.
Open the Events section of the palette, and from the Catch Events group drag a Timer event into the process. While you are still dragging it, move it on to the edge of the User task. It will "attach" itself to the edge of the task. You will see...