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