The CREO fount and the original Aitken fount use a venturi inside the tap.  This is used to either bring in air to beer and create the creamy head that they are famous for, or to bring beer back in through the optional return line.

The science behind this is when the beer pipe narrows the speed of the flow increases, this causes a drop in pressure.  When the beer exits the narrow section back into the bigger area the speed decreases again, the pressure will increase causing a vacuum.  At this point air can either be drawn into the beer via the hole on the front of the tap, or beer can be drawn back on the return line. 

These are the off positions, directly opposite, left and right.

In this position the fount will draw beer in from the return tray and blend it with the incoming beer from the cask.  This is an option, if you are not using returns then you will just get beer directly from the cask.

Putting the handle away from you will activate the air hole on the front of the tap.  You can control how much air is drawn in by putting your thumb over it.  Everyone has their own technique for pouring, how much airiation is required is down to the conditon of the beer coming from the cask