WARNING: NOVEL LENGTH EXPLANATION TO FOLLOW
This topic is extremely complex, and there aren't a lot of clear, simple answers. But I'll give you an overview, and after you see it, maybe you can narrow the focus a little bit in order to get more specific.
I'll assume that you understand how to get a good shoulder and arm motion, since that's beyond the scope of the question. If you look at the full kinetic chain of the serve, that somersault motion of the shoulders and whipping of the arm is the last part of the chain. As you work your way down the body, the core is the preceding part. All good serves get some vertical AND some horizontal rotation/loading on the shoudlers -- which is to say, when you "wind up your shoulders." THAT part of the kinetic chain begins with a pull through the trunk/core. So it goes (in order from the ground-up): pull from the core...unwind the shoulders...whip the arm. All of which sounds irrelevant, but the most elementary footwork starts there.
In the absence of everything else, you want a stance that puts no unnecessary pressure on your joints (esp. knees and hip), and doesn't interfere with that pulling motion that begins with the core. That's the kind of serve that you get when you practice hitting from your knees, and is also the kind of serve you get when you practice what "Serve Doctor" Pat Dougherty calls "The Cylinder Drill." It's a serve that does EVERYTHING as right as possible with the upper body, but pretty much eliminates the lower body stuff. It's also the kind of serve you get if you have otherwise crappy footwork -- and even many pros do this, esp. in the WTA.
This is pretty much the minimal effective serve that uses the kinetic chain at all. It's also what was being referred to as a "pull serve" over in the Push vs Pull thread, recently. It's a serve that either intentionally, or UNintentionally, gets that top part of the chain right, but eliminates the lower parts. It's NOT a terrible serve, necessarily, and is probably close to ideal if you have hip or knee issues. It's also a great starting point for learning a serve.
The next part of the chain, as you work your way down, is the hips. The most elementary way to load the hips as a starting point to the chain, is to "pre-load" them. This ain't gonna turn you into Sampras, but it adds another element of torque to your chain. It's best described by what Pat calls the "spring loaded serve." See below. He describes the footwork in the video, and why it accomplishes what it does.
Everything beyond that is about emphasizing first WINDING, and THEN uncoiling the hips, about getting a push with the legs, and about getting a lean into the court. Again, the more of these things you can achieve, the more potential power and spin you can produce, though virtually all serves fail to capitalize on one or more elements -- it's pretty hard on the body to maximize them all. A perfect chain would include: weight forward into the court -> drive with both legs -> push right hip up and through -> pull with core -> somersault shoulders - whip arm.
And there are TONS of ways to add these elements.
Just to give a few examples: Safin uses footwork that isn't dissimilar to the "spring loaded serve" above, except that instead of "pre-loading" that hip by pushing it forward and then beginning his chain, he manages to keep it wound until he initiates his drive, so it becomes a part of the chain. Notice the similarity: the back foot moves up in front of the foot, and the right hip noticably becomes a power player in the service motion -- just at a later point than in the spring-loaded. He keeps the hip wound, and uses the push of the legs to unwind it. Many people find it easiest to achieve this with footwork that brings that back foot over in front of the lead foot.
A lot of modern players like Isner use a similar motion, but stop their back foot before it passes the front one. This facilitates keeping the hips wound up, but makes it more difficult to unwind them forcefully. Give and take.
Federer uses a platform, and positions both his front and rear feet so that he can get maximum drive up into the ball. The rear foot is in a position that really helps him fire that right hip up and around to start the chain, while the front leg drives mostly upward. In order to get that hard push off the back foot, he compromises his lean into the court somewhat. This makes sense for him, since he is in no big hurry to get to the net. This is why Roger seems to get so much "jump" as he serves: BOTH his legs are being used specifically to push upward -- HARD. This is the way most modern players employ the platform stance, but it's not the only way.
Contrast with Pete Sampras, who on the face of it, uses a very similar motion. But Pete isn't getting nearly the drive off the rear leg Roger is. He can't, because he's using that wide platform more for balance, but is moving his weight WAY further forward. The rear leg physically CAN'T push up against weight that's no longer there. See how far over the front hip he gets? His center of gravity is right over the front foot before he initiates his push. Thus, most of that push comes from that front leg. He compromises a bit on the push of the right hip, but more than makes up for it with his exaggerated forward lean and momentum. See how his rear foot trails behind him, while Roger's pushed so aggressively up? Different ways to skin a cat. Pete's very much invested in getting his momentum moving into the court, so this makes sense.
In practice, it's just about impossible to push hard off that back leg and simultaneously drive deep into the court from a wide platform stance. Because you can't move your weight that far forward, and still push with both legs equally.
I've only noticed ONE player, ever, who managed to really employ every unit of that full kinetic chain via his footwork...Andy Roddick.
He keeps such a narrow platform that he can move his weight WAY forward into the court over both feet, and STILL gets a hell of a push off of BOTH legs. The back one drives his hip through with a vengeance, and of course the rest of the core-shoulder-arm motion is there as well. It's not a coincidence that Andy set records with his serve despite being Federer's size. It wasn't because he was gifted with a golden arm by the tennis gods. It's because he had the most complete kinetic chain. Big lean -> strong push with both legs -> huge hip fire -> upper body. Effing hard to do.
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Recommendation? Practice the cylinder drill till you get really, really good at hitting serves that way, and then experiment. See what lets you fire that hip easiest. See how comfortable you are getting your weight forward into the court. See if it's more natural for you to push off of one foot, or both. Then find a player like those above to model till you get it somewhere near what you like. As long as you understand what each element of that chain is doing, it starts to make sense what your feet and legs are doing, too. There's no one-size fits all answer. Get as many of those elements as you're comfortable with into your motion, and you'll whipcrack serves like you never have before.
It all starts with the feet.
