The "Left Side Lead" is a very important element that allows the two-headed four-legged animal (entity) to move gracefully and efficiently. It is associated with American Style Smooth and International Style Standard Ballroom Dances. It is a key element in the Silver and above levels of the Smooth and Standard dances. This does not, however, include Tango because Tango is danced with a "Right Side Lead".
Let's look at American Style Smooth dancing (Silver and above levels) from a "Left Side Lead" perspective. The "Left Side Lead "is created as follows: The man stands with his feet together. Now move the right foot backwards so as to be in the middle of the left foot. Allow the body to naturally adjust to the new foot position. The lady now gets into "Closed Dance Position" with the man. This will give the partnership a natural "Left Side Lead".
The "Left Side Lead" helps maintain the proper offset of the partnership. In this position, the lady is to the right and slightly behind the man. When traveling forward, the man has a left side forward feeling and the lady feels like she is slightly behind in the man's right elbow.
A key function of the "Left Side Lead" is to close the gap between the individual dancer's legs/feet. Stand with your frame parallel to a wall and then walk towards the wall. As your legs hang naturally under each hip joint, a space will be created between your feet. Your walk will have a slight side to side movement because of the distance between your legs/feet. Now create the "Left Side Lead" as described above. Your frame will now have a diagonal reference with the wall with the left side forward. As you walk towards the wall you will notice that your legs are brushing each other as you walk and there is no side to side movement. A good rule to remember is: "Any time your legs cross, there should be no air between your legs".
Now there is new direction of "forward" and "backward" when you have a "Left Side Lead". Forward and backward is not in reference to the direction your shoulders are facing, it is in reference to the direction you are moving (in this case left side diagonal forward). As the dancer takes steps forward or backward, it feels as if he/she is always stepping toward their opposite shoulder. The dancer should feel the legs brush as they pass each other.
There is also a new counter balance position. The partners are no longer balanced opposite of each other from a shoulder perspective. They are now diagonally across from each other because of the "Partnership Offset" and the "Left Side Lead". This changes the counter balance and head position. When dancing in this position, the partners should barely be able to see each other. It is very important that the dancers change what they are visually seeing (not seeing their partner) with this new partnership dance position.
The term "Shape" refers to the "Left Side Lead" action. It is usually used to describe the amount or degree of "Left Side Lead". A major component of American Style Silver Waltz and Foxtrot is the passing of the legs (instead of closing). The "Outside Partner" dance position is used to accomplish this leg action. This leg action is achieved by creating more "Shape" ("Left Side Lead") allowing the dancer to step outside of his or her partner. "Outside Partner Dance Position" is only from the knee down. The "Shape" used for "Out Side Partner" has a little bit of CBM (Contra Body Movement) to help maintain the partnership. I have described American Style Silver Waltz and Foxtrot because most of American Style is danced in "Outside Partner" dance position.
International Standard Style dancing uses the same "Left Side Lead" to accommodate the partnership and movement as well, but just not as often. Legs passing in International Style are usually called "Continuity Steps". A few examples would be: Continuity Ending (American Style Feather Finish), Lock Steps, Chasse (American Style ends in Promenade, International ends Outside Partner), etc. This is one aspect that makes the two styles different.
"Shape" ("Left Side Lead") is a mandatory skill necessary to achieve quality movement at the Silver and above levels of American Smooth and International Standard dancing.