Although there are moments of rest, tango is mainly in flux. The fluidity of the dance needs to be materialized in the flow as well. Experienced dancers know that the body produces continuous lilts or cadences due to the alternate weight shift from one foot to the other in the dance. (See Cadencia.) When two dancers swing together in rhythm to the music, it feels like a baby being comfortably swayed in the cradle, or fish being gently surged by the wave in the water, which is a very cozy feeling especially for the woman, as she is the one nestling in his arms and enjoying the ride. (See Driving and Synchronization.)
The swing could be mono-directional, but usually it alternates in opposite directions. The man swings the woman's torso, causing her free leg to swing along. As her leg lands on the floor, he goes on to swing her torso in the opposite direction, resulting in her other leg to swing reversely. In order to do that, the woman needs to swivel her hips as her weight is shifting to the landing foot, so that her free leg could take advantage of the inertia of her moving body to swing in the opposite direction. This is known as ocho, the most common step in tango. The cadencia used in both front ocho and back ocho is essentially the same technique. (See Dissociation and Gear Effect.) Often, tango teachers do not emphasize the swing of the body and leg when they teach ocho. However, if the cadencia is blended in, it will not only make the ocho more elegant, outlining the feminine beauty of her flexible body, but also produce a swing or flying like feeling, making the movement 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 any forward or horizontal motion, it would be difficult to generate the swing. That is why skilled 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 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. (See Spot Dancing in Tango.) In an empty room this may cause no problem, but on a crowded floor that is too senseless. Mark Word calls such people "rocks in the stream". You drive to work in the morning and suddenly there is a car way below the speed limit 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, pause for a moment, and then move on. Veterans do slow movements as well. But they do so only when the music tells everyone to slow down, or when there is enough space around. If the people behind are approaching, then you need to move forward to avoid causing obstruction to traffic. This is the codigo, which everyone dancing on a crowded dance floor should know and follow.