Experienced dancers know that the body produces continuous lilt due to the alternate weight change from one foot to the other, and the lilt can be enhanced by increasing the motion of the body. This lilt of the body is called cadencia in tango. When the woman is led to do cadencia, it feels like a baby in the cradle being gently swayed by the parent, or fish in the water being gently surged by the wave, which is a cozy feeling for both but especially for the woman, as she is the one nestling in his arms and enjoying the ride. (See Driving and Synchronization.)
Cadencia is usually executed alternately in the opposite directions. The man swings the woman's torso, causing her free leg to swing along. As that leg lands on the floor the woman changes the weight to it and at the same time swivels her hips to allow the man to take advantage of the inertia of her body to swing her other leg in the opposite direction. This is how ocho is danced. Many tango teachers only emphasize the dissociation or the swivel of the hips when they teach ocho, but overlook the swing or cadencia of the body. However, if the cadencia is blended in, it will not only increase the elegance of the movement, but also produce a lilting feeling, making the movement more enjoyable.
To generate the cadencia, there must be some speed in the horizontal direction. If you stay at the same spot doing steps without a horizontal motion, it would be difficult to generate the swing. That is why experienced dancers like to dance in the flow. When the floor is full of experienced dancers, you will see a counterclockwise flow of people like waves surging forward in correspondence with the rhythm of the music, and the speed of the flow is quite fast. But if there are too many novices on the dance floor, then the speed of the flow is slowed down. Sometimes it even becomes like a pool of stagnant water.
Novices who have no sense of flow often remain at the same place doing steps, disregarding the people behind waiting for them to move forward. (See Spot Dancing in Tango.) In an empty room that may cause no trouble. But if you dance on a crowded floor, that could cause obstruction to traffic. Mark Word calls such people "rocks in the stream". You drive to work in the morning and suddenly there is a slow car in front of you, that is the same kind of feeling. People dancing on a crowded dance floor must not be such "rocks in the stream". I'm not saying that you may not slow down for a moment and then move on. Experienced dancers do that also, but they do so only when there is enough space, or when the music tells everybody to slow down. If the people behind you are approaching, then you need to move forward to avoid causing obstruction to traffic. That is the code, which everyone dancing on a crowded dance floor must follow.