If you don't "get" the fast or power tack right away, don't be discouraged. You're just like everybody else. This page should help you focus on the elements that are giving you trouble.
Nothing is more critical to success than the finish. A decent finish can salvage your tack, even if you've bungled everything else along the way. But without it your tack will fail even if you've done everything else perfectly.
Pay attention to the finish in this continentseven video of Iballa Moreno.
Study it more than once. Think "Sagittarius" (the constellation of the "archer"). In other words, think of someone shooting a bow and arrow. That's the body and sail position she gets into to regain momentum after she's moved to the new side of the sail.
By the way, you can watch other examples of the power tack at the same continentseven website. Although the pros use different techniques for moving to the new side of the sail, notice that they all use the same "Sagittarius" finish.
Next, notice how Andre Paskowski moves his hands to the new side of the boom -- it's an overhand crossover.
Don't blink; he does it quickly. He's offloading his weight from his feet (which are moving) to the mast by putting his weight on the boom. At this point in the maneuver, the mast (which is firmly planted on the board) is a more trustworthy "foot" than his own 2 moving feet. So he gains stability by weighting his "third" foot. The mast acts like like a crutch.
An excellent, alternative way to get your hands to the new side of the boom is demonstrated in this clip as well as some of the other videos above. The video was scraped from the old defunct Google video site; I believe the windsurfer is Lars Petersen:
The key is to grab the mast. Grabbing the mast is an excellent way to feel where the wind is; you should find that this "feel" improves your sense of 12 o'clock on the windclock. It also lowers your center of gravity so you're less likely to topple over. By the way, the finish in this video doesn't highlight the Sagittarius (bow and archer) position as well as the videos above do. He's still doing the archery thing, but the camera angle doesn't make it as obvious.
Try the hand crossover both ways -- overhand or grabbing the mast. There are other ways too! You can cross underhanded with your palm facing up. Or you can shuffle your hands along without ever crossing them at all. None is "right" or "wrong." What counts is what works best for you. Experimentation is a good thing in windsurfing.
When should you make make the big move to the other side? You might find it reassuring to wait until the board has lost its forward momentum, so that it's nearly at a standstill. But watch the videos above again. The pros make the move much earlier, while the board is still travelling forward at a good clip. As you increase your confidence in the maneuver, you too should move earlier. Why? Because like a bicycle, a moving board is more stable than a stationary board. How should your feet move? First place your foot in front of the mast, as you would to initiate a beginner tack. It's even OK to get a little head start by nudging it a bit further around, so it's on the other side of the mast. Then take 2 quick steps to get your body to the other side:
Watch Iballa Moreno above one more time and notice the lag that occurs from the time she is on the new side to the time she pushes the sail forward into the Sagittarius (bow and arrow) position. How long? For at least a second she hangs out on the new side -- steadily -- before actually heading downwind to complete the tack.
How does she achieve this tranquility? By backwinding. She moves early to the new side, before she reaches the eye of the wind. That means that when she arrives on the new side, the wind's force is still on the old side of the sail. She pushes against that force for steadiness; no delicate balancing act required. Only when she's good and ready does she push the sail forward into the bow and arrow position to capture the wind on the new side, turn downwind, and sail off in her new direction. Once you get comfortable with backwinding you'll find you can even relax your back hand and backwind with only your front hand on the boom.
Meredith Robert of of ABK Boardsports showed me this backwinding trick; she also pointed out that Iballa was doing it -- something I had not previously noticed.
Here's a good example (also from the old Google video site) of a tack that is decidedly not what we call a fast tack.
Temporary technical difficulties -- this video will be up again soon
Look familiar? It's very nearly what we taught you in beginning windsurfing at Hoofers. Why isn't it a fast tack? Because he moves to neutral position ("midnight" neutral) during the maneuver. Unlike in the fast/power tack neutral position exposes the sailor for a longer time to the vagaries of big winds and waves. To do a proper fast/power tack, don't do what he does; at no point during the maneuver should your feet straddle the two sides of the board. Practice the proper footwork (and handwork) on land in your bedroom or at the bus stop! Here's the moral of the story. When you make the move, make it. Commit. Don't hesitate. Otherwise you'll find yourself only part way around, wobbling side to side. To prevent wobble, commit.