Does nothing to steering geometry. Under the triple, the bars put slightly more of your shoulder weight on the bars and wrists. Great for leverage, plus your body is lower for aerodynamics. Above the triple, the bars put less weight on the wrists, and it's easier to modify. Mine is above, and I raised it 3/4 inch. Best for street riding.
I know that Keith Code (Twist Of The Wrist) build a bike that demonstrated how placement of the bars and counter-steering worked, but I can't remember if I saw it in a magazine or online.