basicly on flatwater you could ride a board on one foot. The essence of freeriding is a free ride, just simple blasting, doing whatever you feel is good. A jump here and there, if your foolish enough you try to pop a move or something like it. The idea of simply blasting is the purest base of windsurfing: you put your weight forward to increase planning surface and you lean backwards hanging onto the sail with the maximum of your wieght the wind can counter thus, at one point reaching the planning stage (the photo you gave us it just the stance). once you've reached planning speed and are into the footstraps boards accelerate because your position becomes ideal: more weight on the back whilst, because of the sailpressure that weight is minimal. This in combination with a correct fin gives a hell of a ride. But then, if you stick to this position, something always goes wrong and you end up catapulted (because there is little grip on the board at this point one little windgust does the trick) so you have to change position: your stance becomes more upright (mind, your still going downwind here) and more strait legged (usually both legs have a parallel bend in the knee) with still maximum front foot pressure.
at this stage you wanne go upwind, blasting downwind is fast and i can be dangerous if you don't know what your doing. Upwind is a more secure direction of sailing and you don't end up drifting of like a madman. So what do you do now? you have to do a few things: one is to move the sail backwards giving you a different sailing direction. At the same time you want to hang into the sail again almost to your max and pull your toes up and create a more neutral stance, there is usually weight on both your feet now and in many cases both legs are streched. Once you have your sailing direction the position tends to change again because at this point too much backfoot will kill. So you get into your stance: lean forward, bend through front leg, and curl up your toes.
As you can see there is a sort of metamorphoses going on through every step of the planning way. This isn't something that is tought to you, you can feel it. The sail pulls you into these stances and by applying the right kind of back and frontfoot pressure you can maximaze or minimize your speed as much as you like.
hope this helps
Haiko, AKA crazychemical