A Place Diverter is basically a trim device.
I does exactly what it sounds like it does. When in the full down position, it forces the bow down (or the transom up if you prefer) this helps get the hull "up on step" or to help generate a plane at lower speeds. It wont be fast, quite the contrary, if too low, the steering will feel heavy and exaggerated.
When the hull is planing (lets call it 35 MPH just for giggles) then you can raise/lower the nozzle to "tune" the ride, slowly begin raising the setting and you will feel the hull "free up" or "release" from the water. The side spray will also begin to move rearward. Too high and the bow rises to the extreme and speed begins to fall like a rock, also the boat can tend to porpoise, this is the uncontrolled bounce you can sometimes get that is very annoying.
At much higher speeds (call it 55mph+) the same movements hold true but to much more of an extreme, this is where a Diverter really shines.
Most 18-20' Jets like about a 6' tall rooster, this is just an easy way to tell where you are at as far as tune, some boats like more, some less. A droop can help or hurt in this area too.
One thing to keep in mind when driving at speed with the Diverter up is that steering can be compromised somewhat and the boat will not react the way you think it will. It is important to try driving it in different conditions until you get the feel. Especially with an area free of F-tards.
Hope this at least gives you some insight or at minimum, makes for more questions...
GT