Hi Tim
As the water level raises in the OF compartment, it also raises in the turned down part of the Durso. When it reaches the height of the horizontal section it flows into the turned down section, which is the long pipe that stands up. It then begins to drain.
The Durso is a great standpipe, but I prefer a modified version of the stockman stand pipe. I eliminate the top cap and just put the reducing bushing all the way up to the top of the pipe. Then I cut a piece of acrylic (anything flat will work) to cover the top of the stockman. Drill some small holes in it and use a gasket between the acrylic and the standpipe. Then I use rubber bands to hold it in place. What this does is allows you to set the water level just high enough to achieve surface skimming, but not protrude above the tank. Yet still eliminating the noise of water falling over the baffles. As well as being able to remove the cover for periodic cleaning.
Here's a pic(not so professional looking graphic)