What is Swing. The dictionary says . To move freely back and forth or side to side while, or if as suspended. I have been one of those who was taught in the Waltz to move to the side on step two, body and all. Then I was informed that that's ok for a low grade medal. But to think of a Grandfather Clock with a pendulum. If the pendulum swings to the side and the above ( called the fulcrum point) moves at the same time and the same speed the pendulum can't swing, neither can we. Sway. Because the weight is over the standing foot, which includes the head and we are swinging, the fulcrum point wll be passed over to the new standing foot,( we don't want to swing in reverse), so we move over where swing will join sway. The other way I was as flat as a tack, rise to a mimimum with a very forced Sway.

In my understanding, The speed is changing .On the later half of last beat of previous bar, we gather the energy to drive body weight away , and swing our first step into its position on 1, this is the duration of fastest speed , then we swing up gradually during the remained duration of one bar, the speed ---> alomost zero ,at up point, then foot flat , and repeat another swing action. My point is that :swing presents and continues in whole figures , 2-4 steps included and its speed is changing. Sway is a result of swing (other poise change called inclination )resulting from our body weight distribution: upper part including head weight is still over the old standing ball-toes (< 50% of BD weight ) ,for countra balance and beautiful body line.
Body weight distribution while foot passing is interesting. Some alteration may happen when some factors are involved such speed of music , dancer's style , the figurs etc. ( See Luca 's dvd ). Very early, I have ever asked about the relationship of body speed and foot speed. In fact , one rule can't cover all situation . Receiving foot absorbs our body weight not reject it , our knee and ankle must be softened / flexible in right time (especially the poment of 100% support BD weight and body weight passing points just before across the floor ) ,receiving foot controls our moving rhythm as we want , not the old stnading foot. Generally, body moving commences first , foot speed more fast to receive it. So , we have energy to drive BD weight moving on dancing trace without body falling , I concluded 3H for myself : half beat, half stride, half weight to test my posture, body weight movment , timing. It really helps me to move smooth and stand stably in most figurs I learnt. Now I have more concentration to uninterrupted moving technique and extend my shpae responding to man's body signals, adjust my posture, care my axis, listening to melody etc. There are a lot of knowledge to learn . Quite good.
ylchen