The supporting leg is the ultimate BOSS in Ballroom Dancing. It is a key element in controlling movement and communications within the two-headed four-legged entity. The supporting leg is the gateway to achieving Ballroom Dancing at the highest levels. The supporting leg initiates and communicates the action to be performed.
Moving from the supporting leg is important at all levels of Ballroom Dancing. In the beginning when dancers are starting out, they move within themselves instead of moving as a two-headed four-legged entity. They typically move from their free leg. The power of movement must be initiated from the supporting leg (the BOSS). "The follower can only feel the movement from the supporting leg. The follower cannot feel the free leg move."
The basic lead for Waltz is lowering on, then powering from, the supporting leg on the "One" beat. When movement is initiated from the free leg, it feels as if the frame is pushing the partner. Lowering on the supporting leg, engages power from the partner. This creates power from both dancers to initiate movement.
In Samba, there is an expression describing its basic movement: "Down, A, Down" (1, ah, 2). Let's look at the transition from a "Forward and Backward Basic" to a "Side to Side Basic". The second "Down" (2) is where the transition takes place. At this point in time, the leader has the option of changing the direction of power from front to back, to side to side. The down action allows the Supporting leg (Boss leg) to be able to push off and power to the side. Dancers often try to make this transition by moving through the free leg. When the free leg is used the lead is late and the partner feels like she is being pulled to the side.
In Ballroom Two Step, there is a dance element called a "Left Turn" (Slow, Quick, Quick). In this element there is a crossing step on the "Quick, Quick". The leader's left leg crosses in front of the right leg. Unfortunately, many dancers just step across with the free leg. This leaves the lady lagging behind during this action. If there is a little foot rise at this point, the leader can very directly lead the crossing step. The supporting leg can pull the free leg through. This includes the ladies crossing free leg.
A similar action happens on "Lock Step" elements (Standard, Smooth or Latin, Rhythm). On forward locks, the front leg (Supporting leg) pulls the free leg into the lock. On back locks it's the back leg (Supporting leg) pulling the free leg into the lock. This allows the follower to feel the intended action instead of guessing.
Another little different action of the Supporting leg is Waltz "Leg Swing". In Silver and above Waltz, the free leg swings from behind the dancer through to the front of the dancer while on rise. This action is powered by the supporting leg powering up. The more upward power, the faster the leg swings through. I like to call the action of the power step followed by leg swing, "Power and Glide". This "Power and Glide" action gives the dancer more control of the movement.
The Supporting leg is involved in almost all four-legged movement. This is not something you can just turn on when you dance. You need to make this part of your life. Use it all of the time. The following is an exercise used to develop the feeling of always moving (powering) from the supporting leg. In Closed Dance Position with a partner, walk forward imagining that you are walking up a slight incline plane. If this were actually true, you would have to push off (power) on every step. Now, have your partner give you a little resistance as you walk forward. This creates a real resistance that forces the supporting leg to power the movement. As long as the leader initiates movement from the Supporting leg, the follower will be able to respond properly and on time. This action is imperative to unify the two dancers into a single moving entity. Using the BOSS leg will improve your four-legged movement immeasurably.