Experienced dancers know that the body produces continuous lilts due to the alternate weight change from one foot to the other. The lilt can be deliberately enhanced by increasing the swing of the body. The technique is known as cadencia. When the man leads the woman do cadencia, it feels like the baby being comfortably swayed in the cradle, or fish being gently surged by the wave in the water, which is a cozy feeling for her 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, he goes on to swing her torso in the opposite direction, resulting in her other leg to swing reversely. The woman may take advantage of the inertia by swiveling her hips as her weight is sifting to the landing foot to let her other leg swing in the opposite direction. This is how ocho is danced. Many tango teachers emphasize the swivel of the hips but overlook the swing of the body when they teach ocho, But, if the swing or cadencia is blended in, it will not only make the movement more elegant, outlining the beauty of her flexible body, but also produce a lilting feeling, making the dance more enjoyable.
To generate the swing, 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 the flow of people moving counterclockwise like the waves surging forward in accordance with the rhythm of the music, and the speed of the flow is quite fast. But if there are too many novices on the 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 spot doing steps, disregarding the people behind waiting for them to move. (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 blocking your way, 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 cannot slow down for a moment and then move on. Veteran dancers do that, too, but they do so only when there is enough space, or when the music tells everyone to slow down. If the people behind you are approaching, then you need to move forward to avoid causing obstruction to traffic. This is the code, which everyone dancing on a crowded dance floor must follow.