The goal in Ballroom Dancing is to move as "ONE" on the dance floor. The two dancers (partners) become a single entity (a Two-Headed Four-Legged Animal). "Leading" is the action that accomplishes this transformation. Leading is the act of moving in such a manner that your partner responds at the same time and with the desired action.
When I went to Air Force basic training, one of the first things that we learned was to march. Marching was the method used to move a large number of recruits from one location to another. The recruits were assigned to a Flight (a group of 80 recruits, for example: Flight 105) and the LEADER of the Flight was the TI (training instructor). He leads the Flight of 80 recruits by issuing verbal commands. At this point in time the untrained recruits have no idea of how to FOLLOW the commands. Moving a group of new recruits to a new location without having any marching training is like watching the "Keystone Cops" (complete confusion and disarray).
The first command learned was: "Flight 105…Fall In". This means to assemble Flight 105 in a designated area and line up 20 recruits across, 4 rows deep, an arms length from the person in front of you and an arms length from the person to the right side of you. Your cap is positioned on your head with the visor 2 fingers above the nose. This keeps your head up in the correct position while marching. Once Flight 105 is assembled, the command sequence was: "Flight…Left…Face". Each recruit pivots on his left heel and right ball then closes the right foot to the left (Every one turns to the left). "Flight…Forward…March" The whole Flight starts marching forward starting with left foot taking heel leads. "Flight…ToTheRear…March" The whole flight takes one left step forward then a right step forward pivoting on the ball of the right foot turning 180 degrees continuing to march in the new direction. "Flight…Halt" The whole flight takes one left step forward and closes the right foot.
Let's look at the command: "Flight…To The Rear…March". The word "Flight" alerts the whole Flight that a command is about to be issued. The words "To The Rear" tells everyone what to do. The word "March" tells the recruits when to do it. The "March" portion of the command is verbalized by the TI when the recruits are stepping on their right foot. This allows the Flight to start executing the command with their left foot as described above. It is very important that the recruit understands the rule that the command is executed on the next 2 steps after the command is issued.The point of this discussion is that marching has specific rules, a special command set and the timing of the verbal commands is crucial. I always talk about the Ballroom partnership as being a Two-Headed Four-Legged Animal. The marching partnership is a 10 foot by 60 foot, 80-Headed 160-Legged Centipede. The TI is the LEADER and the 80 recruits are the FOLLOWERS. You can see that specific rules are required so that all the members of the Flight can move under control, in the proper direction and in unison. Each member of the Flight must be trained in the rules of the commands and the TI must be trained in the proper issuing of the timing of these commands. If only 1 of the 80 recruits executes the command incorrectly, it will be very obvious which one and you really don't want to be that recruit.
Ballroom dancing is actually much more complicated. We are dealing with only a two-headed four-legged animal but there are no verbal commands. Instead, there is a physical communication through the partnership connection that creates the lead and follow. The LEADER and FOLLOWER needs to be trained in the rules of Two-Headed Four-Legged movement to become a proficient Ballroom Dancer. This is what I call a "trained dancer". It seems that marching is more complicated because of the 80 recruits involved, but marching has just one style. Ballroom partnerships are just 2 dancers but the lead and follow is very subtle and communicated through feeling. Ballroom Dancing has many styles: Latin, Standard, Rhythm, Smooth and all of the dances within each category (Waltz, Rumba, Tango, Bolero, etc.). There is so much more to lead in Ballroom Dancing. Being a trained dancer becomes even more important.
The leader needs to be able to create the specific feeling necessary for the follower to be able to identify and respond correctly. The leader must know the followers part and how the follower will respond to a specific lead. The style of the dance must also be lead (up and down movement in Waltz, smooth even movement in Foxtrot, Latin Motion, staccato movement in Tango, etc.). The timing, the footwork, rise/fall, shape of the partnership, amount of power applied, head position, etc., are all parts of the lead.
The follower needs to be able to identify the feeling of the lead and to be able to respond correctly. There are other partnership concepts like "Continuation of motion", "Mirror Image" and numerous others that have to be learned and understood as well.
It is obvious that most of the above can not be learned in Ballroom Group Classes. If you only take Group Classes, the best you can hope for is becoming a "Pattern Dancer" (dancers walking through learned patterns in front of each other).
Leading and following evolves over time. You don't just all of a sudden have it. Your instructor needs to give you the "feeling" so you know what you are after. At the highest levels, both partners must be "trained dancers" to get the best results.
The very first rule that I learned in the military was known as: "Rule #1": "Know what you are doing." This is a very good rule to follow in Ballroom Dancing and in every aspect of your life.