When I first started dancing, I had the impression that the lead came only from the "frame". Kind of like steering a car. As I progressed in my dancing, it became clear that the lead came from many different places. But the "frame" is the main conduit for the partner to be able to "feel" the lead. Without a good "frame", there is no lead. I tell my students all of the time: "97% of what a lady does wrong is because of the "man's lead." The lead is wrong, too slow, too fast, an incomplete lead, or just plain no lead. The lady also needs to know the correct response to a specific lead (This is the other 3% of what she does wrong.). This is what a "trained dancer" is all about.
A good leader knows what the lead is for what he is trying to do. This means that the leader must know what he is trying to get the lady to do not just know what he is supposed to do. A good leader knows the ladies part.
Most men have the expectation that the lead is a very overt action, when in fact, most of the time it is very subtle. This is why a lot of men over-lead and the lady feels like she is being pushed around instead of danced. As the man's lead progresses, he will start dancing the lady, not himself. This is where it starts feeling really good!
The lead comes in many forms. One of the most basic leads is simply being on only one foot at a time. When the leader has his weight on both feet, the lady can't feel which foot she should be on. The lady can only "feel" what is happening through the supporting leg unless the man's free leg is touching the ladies free leg. This is why just about all of our movements are initiated from the supporting leg.
There are basically two types of turns in partnership dancing, powered turns and un-powered turns. In either case, the leader determines the speed of the turn (ex. 1,2,3 or 1,2,&,3 or 1,&,2,3). In an un-powered turn the lady feels the man's weight changes through the hand connection (frame), which controls the speed. In a powered connection, the power in the hand connection determines the speed of the turn.
The shape of the leaders body can cause the lady's leg to cross in front of her body or behind depending on the shape. This is a major aspect of Advanced Waltz and Foxtrot. A more simplistic example would be the crossing step in a left turn in Ballroom Two Step. The lady's leg crossing in front is lead by body shape causing the lady's right hip to be slightly forward of her body allowing her right leg to swing across in front of her body. The man's shape determines that the leg moves in front of her body and a slight rise and fall accommodates the leg swing.
As you see the lead comes from many places and this is not a complete list by any means. The goal for the man, is to have such a good lead, that there is only one action that the lady can take. The lead should be such that the lady moves naturally and there is no decision for her to make.