Jib fairlead positions are similar to a three speed vehicle
transmission; 1st gear in a car being similar to the forward track setting for a
fairlead, 2nd similar to the middle position, and 3rd similar to the rear-most
position .
For this example, 1st gear (blue), 2nd gear (yellow), and 3rd gear (green), are formed when an imaginary line is drawn from the fairlead block through the clew and then through the luft. *(In your mind's eye, draw a straight line outwards using the jib sheet.)
Although sometimes modified by the designer for better sailboat performance, the middle block position (yellow) divides the luft directly in half. The foreward block position divides the luft towards the upper portion of the luft. The rear-most block position divides the luft towards the lower portion. This will always change if you modify or rake your mast differently.
As your boat accelerates, the power from the sails tip your boat forward. The bow essentially dives in the water. This "dive" allows more water to be pushed aside which adds up to more resistance. Eventually your boat speeds up and the plowing effect lessens. *(The difference from sail power to boat speed gets less and many boats either plane-up or lift as the speed increases.) Sure this is generally a small thing but if every second counts…
First gear, (Blue in picture).
To lessen the "dive" effect on your boat, move the jib's block forward. This puts more tension towards the head of the sail and in doing so, is allowing the foot to become loose. That lets a larger portion of the wind to be directed towards the bottom. The force from the downward blast of wind starts to lift the bow which in turn is lowering the resistance and lets the sailboat accelerate quicker. (Think of it as a sailboat wheely!)
Second Gear, (Yellow in picture).
Now that your boat has started out quickly, you'll need slightly more rearward wind to increase speed and not letting the bow down. By sliding the jib's fairlead to the middle you'll be tightening the foot slightly and allowing equal amounts of wind to be directed both downward and rearward. *(Boats that don't have an adjustable jib track are often set in this position by the factory as a good all around average.)
Third Gear, (Green in picture).
The faster the boat goes, the more rearward wind you want as less bow lift is needed. Think of this position as shutting off downward wind. Third gear tightens the foot and letting most of the wind escape the jib towards the rear. You'll be getting the most foreward push in this position.