Flo'da--
Check out around 2:50 of this vid from Gatlin's 200 in Monaco:
https://www.youtube.com/watch?v=UcGjy6M-7x8
It's the finish of the 200, with Gatlin reaching for the line. Look at his shoulders, they do not remain square to the track, but instead, he is twisting his upper body. This is generally referred to as "rocking", and is usually seen as a bad thing, something that often goes hand-in-hand with overstriding.
Gatlin, however, is different, he uses it just enough to be able to lengthen his stride. When you run/sprint, because your limbs are outside the centerline of your body, when they move, you create a torque. That torque needs to be balanced by something--for instance, an arm swing. However, the torque can also be balanced by a twisting about the body centerline. You see different guys do this in different degrees. Gay, as I mentioned before, does it progressively throughout a 100, IMO perfectly, just staying below the point of overstriding while maximizing stride length. Guys like Lemaitre and Ndure do it more and regularly because their arms aren't as big and can't produce as much of a moment, so do properly balance the legs, they need to twist to produce sufficiently balancing torque.
For an excellent example of the timing I'm talking about with Gay, watch how he unfurls pivoting at the end of his excellent 9.69, as he flies through the end of the race--watch his shoulders pivot increasingly more until he finally crosses the line, starting at about 1:55 in this vid:
https://www.youtube.com/watch?v=MhsqVfOB9ig
As a good comparison between 2 different sprinters, check out this slow-mo front view of Vicaut and Lemaitre side-by-side, at about 4:23
https://www.youtube.com/watch?v=gpX4lPRyP2M
They are both pivoting, but clearly Lemaitre is doing it much more than is Vicaut. Lemaitre has neither Vicaut's upper body size nor strength. Because he doesn't have the strength, he can't move his arms as fast, meaning less acceleration, meaning less force, meaning less torque. Even if he could move them at the same speed, the torque would be less because the mass would be less. So, he needs to pivot more, and he does.
This pivoting enables you to swing your leg farther. Because the torques have to be balanced as they are produced, you need to apply the balancing torque at the right time, in order to enable high knees during the appropriate times of the sprint. Why are high knees good/necessary? Because true sprinting is more akin to a bounding motion, than to what people understand as a running motion. It's all about foot rearward velocity at the point of footstrike, and that is produced by getting the leg into the right position at the right time, leaving enough time to drive it rearward...and that right position is high knee lift, because it gives you enough time to develop the force needed for max rearward foot speed.
Check out around 4:13 of this vid of Powell's excellent 9.72
https://www.youtube.com/watch?v=ZEQrg00Y0dQ
Even Powell does it. Everybody does it, to some extent.
Gatlin, however, is the master of the rearward leg sweep. Ever wonder why he has always been known as the "long strider"? It's because he has a longer arm sweep, and he combines that with just enough pivoting. He is able to produce more balancing torque than other sprinters, and is therefore able to both produce greater rearward foot speed, and apply more force to the track. Look at the end of that 200 and compare his stride frequency to that of Ashmeade--much lower. It doesn't even look like Gatlin is going that fast, but he is.
Of course, one can over-do it. If you do it too long, you will have too great a hang time, and your body will actually get pulled down too far by gravity, such that you now need to apply too much of your force in the upward direction, and less in the needed forward direction. This is IMO what produces overstriding--your body instinctively knows when it need to contact the foot with the ground in order to keep you upright, after you have sank a certain distance. Overstriding occurs when your stride takes so long that the foot has to contact the ground too soon--that is, before the foot is right below the body. When you do that, your foot has not reached maximum rearward velocity, and you are retarding your motion, and decreasing your speed.
What pivoting about your waist really allows is for your pelvis to pivot--your hips. You hear sprint coaches all the time telling sprinters to drive with the hips on acceleration, but WTF does that mean? IMO it means that the hips, or pelvis, should be the prime motion of consideration. Focus on the hips/pelvis, and on what they are doing. Get the right rotation, to get the legs/abs/back into the optimum power position. Focus on what the hips are doing, and the rest will follow...BUT, in order to get the hips to do the right thing, you need to produce the right action in them, and that's where arms/torso pivoting come in.
Powell, again. One of my favorite starters. Watch the slow-mo front view starting at around 3:18 of this vid of his excellent 9.78 in Rieti:
https://www.youtube.com/watch?v=V_tp1V3ikbQ
He has GREAT, rangey arm action, enabled by his torso pivoting. How does torso pivoting enable better arm action? The same way that pelvic pivoting enables better leg action--because both the shoulder joint and hip joint only permit a very limited amount of rearward action by the arm and leg respectively. The joint, be it the shoulder or the hip, need to be pivoted, or tilted (in addition to the limb being rotated), in order to get greater range.
This can be easily illustrated: for the hip, stand straight up, with your feet pointing straight in front of you. Put your hands on your hips so that you know what they are doing. Try to sweep your leg rearward while keeping the foot pointed straight ahead, and see how far it goes. Not far at all. Now, simultaneously pivot at the hip and turn the leg outward a bit, and you will see that it goes much farther.
For the shoulder, just stand there with your hand flat down at your side, and your elbow in at your side, and try to sweep your arm back while not moving your shoulders. It hardly goes back at all. Now you can either bend forward at the waist(as on acceleration), or twist the torso a bit, and you will see that rearward range of the arm is much increased.
To see where this can lead, look at Gay's rear arm position in these still photos:
http://www.dailymail.co.uk/sport/olympics/article-2180394/London-2012-Olympics-Tyson-Gay-quiet-American-ready-blow-Jamaican-dream-apart.html
Back to Gatlin, check out the front-view slow-mo at 1:44 in this vid of his amazing 9.80(+0.1):
https://www.youtube.com/watch?v=RyfEIerlk0Y
Perfect. He is pivoting perfectly in the upper body, too bad you can't see his lower body in the vid. He stays nice and low to the track to avoid floating. Rodgers has started to emulate this action. Watch the slow-mo head-on view of the 2 of them in Linz this year, at about 1:29
https://www.youtube.com/watch?v=ayB5jHuuTac
Rodgers is running with more turnout in his legs, like Boldon used to do. His hips are lower to the ground, and he is pivoting and extending more than he used to--and is going faster as a result.
For me, focusing on this during all phases enables me to focus on generating speed down the track, on covering ground, on eating up ground quickly, which is what it is all about. Not only on acceleration, but especially on SE, when your force production ability is likely to have decreased, and you therefore need more time to accelerate your foot to a high rearward vmax. The bonus is that you don't tie up. I am able to do 200's now in practice, not going 100%, as fast as I could race them 2 years ago, and not feel spent. Incredible! It doesn't feel fast, it feels like Gatlin looked in the 2nd half of his 19.68--in fact, it feels easy. For me, it really works.
Similarly, thinking about it in a 100, during all phases--from acceleration to transition to vmax to SE, unfurling while nearing the line--makes 100's a lot easier. All the force I now apply seems to be going to the right place, and I eat up ground faster. I have combined this focus with increased back-side weight work, adductor work, and hip/core work, and wow.
Back-side weight work: squats from a seated position, tons of ham curls, step-ups, leg press intentionally not using quads at first, with feet out in front
adductor work: huge leg press
hip/core work: hard torso pivots with 135 on the bar (I do not recommend this to anybody), straight leg raises to vertical, bent-knee situps, crunches, etc.
in addition to cleans, but less weight, better form--185 instead of 225, and more reps.
Hope that covers it Flo'da, YMMV. For me, it has worked wonders. Faster than I've been in 5 years, AND uninjured. I'm going to see how long I can keep the development going before life events take over again. I'm trying to avoid being folded-up in airplanes and cars, that kills me and negates much hard work.