Since the inception of ballroom dancing
dancing there have been very few changes. The footwork is the same. the alignments have not changed. The rise and fall is still the same. There have been many changes in the variations and the introduction of new steps. Apart from that the biggest changes have been in posture. Which has little to do with the man. He still stands erect with his arms raised. The changes are, and they are enormouse, is the position of the lady and the use of the knees of both partners. I have seen a picture of Victor Silvester from the back. The only part of the lady not hidden by the body that could be seen was a little bit of the forehead over his right shoulder. Compare that with today.I suppose with the introduction of new steps it was found that the position of the bodies had to alter for obvious reasons. And even today compared to seven years ago there have been changes. Why i say this is because i met up with someone i knew who hadn't danced or kept up with the latest trends for seven years. I showed her the positioning of the body and the contact point which in her day was a heavy contact at the hips. This is so much easier to follow was the words she used.. Anybody add to that.

You mentioned things like the footwork not changing. Better to say that the rules of what-causes-what behind the footwork haven't change, but the footwork actually danced has, because it had to in order to maintain the same rules of causality when circumstances changed.

You noticed that the usage of the knees has changed. That's a perfect example - if you lower more in your knees, for the feet to work the same way they did previously, they will not have to do things a little bit differently. With more lowering in the knees, the rise and fall in the feet is constrained to only the upper part of the overall rise and fall rather than being the majority of the overall. As a result, you'll see in many instances that the heel goes down sooner in the overall course of the step, becuase the touchdown of the heel is no longer the end of the lowering - the lowering continues long after, in the bend of the knee, and of course we want the completion of the lowering to bear about the same relation to the end of the step as it did in the days when we weren't using our knees so much.

Different circumstances + same rules = different details.

This has not changed. Support yourself if you have to. The moving leg the right leg has passed the standing leg and the heel of the standing leg is released from the floor. On the moving leg we have a ball which changes to a heel. We are now at the extent of our stride. We move our weight forward as our weight arrives our knee bends. It is the arrival of the weight which causes the knee to bend and the toe to lower.There are possibilities here. Does the toe lower before the knee followed by the body. Or do we roll that foot lowering as the body arrives. This can easily be simulated by having the hand on a chair and rolling forward and backwards using the whole of both feet.