One of the primary goals of Ballroom Dancing is to become a single entity (the infamous two-headed four-legged animal) moving across the dance floor. We are trying to combine two physical bodies into one. This is relativity simle until we try to move. The legs need to move in a variety of different ways to accommodate the movement of the Ballroom Partnership.
First of all, what is a leg? The dictionary definition of a leg is: "A limb or an appendage of an animal, used for support or locomotion." This is exactly what we need to move this partnership across the dance floor. The typical dance partnership (the two-headed four-legged animal) has four appendages (legs). But let's look at this in a little different manner. Let's count the legs by function, not by physical numbers. For example; almost all of us have seen or participated in, what is called a "Three-Legged Race". Two people, standing side-by-side, facing the same direction, bind the two legs together that are next to each other. The object is to run a race with three legs. Each partner has an outside leg and a common middle leg. The bound leg has a single function, so we treat it as a single leg. That is why it is called a "Three-Legged Race".
Now, lets' consider a natural four-legged animal such as a Cheetah (the Cheetah's leg movements are more obvious). When a Cheetah WALKS, its' left front leg and its 'rear right leg moves at the same time. The left leg supports the left front side of the Cheetah and the right leg supports the rear right side of the Cheetah. The right front and left rear legs move in the same manner. Clearly each leg has a single support function; therefore, the Cheetah is moving in a four-legged manner.
When a Cheetah RUNS, its' two front legs move at the same time and its' two rear legs move at the same time. The Cheetah's two front legs move together and act as a single support (as if they were bound together) supporting the front of the body. The Cheetah's left rear leg supports the left rear side of the body and the right rear leg supports the right rear side of the body. The two front legs (acting as a single support) move in between the two rear legs. The Cheetah is moving in a three-legged manner when it runs. Most natural four-legged animals walk four-legged and run three-legged. Let's apply this same concept to the three Ballroom Dance Positions, "Closed", "Promenade" and "Outside Partner".
In "Closed Dance Position", the partners are standing facing each other. The two legs that can touch (one from each partner) are treated as a single supporting leg. We could bind these legs together to form a single two-legged animal. In "Closed Dance Position", the partnership moves as a two-legged animal.
In "Promenade Dance Position", both partners are facing the same direction with the ladies left leg and the man's right leg touching and the other two legs just slightly apart. If we bind the two touching legs together, we get three legs. In "Promenade Dance Position", the partnership moves as a three-legged animal.
In "Outside Partner Dance Position", the lady faces one direction and the man faces the other, slightly outside of each other. In this instance there are no common supporting legs that we could bind together. In "Outside Partner Dance Position", the partnership moves as a four-legged animal.
Each partner has to understand that the partnership movement is different depending on how many legs are used. If we were running an actual three-legged race and our legs were physically bound together; each of us can't run the race as a normal two-legged animal without someone getting hurt. We have to account for the bound legs and treat them as a single entity.
This isn't as complex as it might seem. Simply move the two legs that are touching as one. We don't need to identify it at two, three or four legged movements. Usually all it takes is awareness by both partners that there are two, three and four legged movements.
The two and three-legged movements are primarily in Smooth and Standard dancing. Most Latin and Rhythm dancing is because the dance partners are farther away from each other and we couldn't bind the common legs. As normal there are exceptions. Bolero is one of these exceptions. Bolero uses a Smooth frame to accommodate the constant rotation of the dance there by allowing leg contact.
I always refer to the Ballroom dance partnership as being a "two-headed four-legged" animal. The "four-legged" is referring to the four appendages. The Ballroom dancer has to accommodate these four appendages weather they are used as two, three or four legs. Moving in a four-legged manner does not work very well when you should be moving in a two or three-legged manner.