I like to think of ballroom dancers as having two bodies: a Rhythm/Latin body and a Smooth/Standard body. Think of a dancer as having just two parts, the body and the legs. The two bodies are not the same because they move differently. For the sake of simplicity I will differentiate between the two by referring to them as Latin bodies and Smooth bodies.
The Latin dancer's body is defined to be from the bottom of the ribcage and up. This includes the ribcage, shoulders, arms, hands, neck and head. The Latin dancer's legs are from the bottom of the ribcage and down. The Latin dancer's legs include the waist area, the hips, the upper leg bones, the knees, the lower leg bones, the ankles and the feet.
The Smooth dancer's body is different. If you take a "Barbie Doll" and pull off her legs, what is remaining is the Smooth dancer's body. The legs would come off at the hip sockets. The Smooth dancer's body would include the pelvis, hips, waist area, ribcage, shoulders, arms, hands, neck and head. Smooth dancer's legs are from the hip sockets down. The Smooth dancer's legs are only the upper leg bones, the knees, the lower leg bones, the ankles and the feet. The Smooth body has an additional feature. This time we take poor "Barbie" and cut her in half at the waist line and install a "Lazy Susan". This allows the Smooth body to rotate in a very controlled manner above and below the waist line.
One of the goals in partnership dancing is to keep the body as still as possible. This allows the two-headed four-legged partnership to be maintained. This does not mean that the body is not working. It is working hard to be still. This is a very important concept. Read the following carefully: "Allow the body to be still; do not hold the body still". An example of this is the problem that many dancers have (primarily men) of dancing through the shoulders. This seems to be true because the frame connects the partnership through the arms and shoulders. The focus is then on the physical connection instead of the logical connection through the center of the partnership. The tendency is to lock the shoulders to hold them in place instead of just allowing the shoulders to be still.
The Latin/Cuban motion of the Latin/Rhythm dances is isolated and emphasized when the body is kept still. When the body is flopping around, it is hard to have precise Latin motion and the partnership suffers. In Latin dancing, all of the emphasis is designed to be from the ribcage down. As in everything, there are exceptions. At higher levels there are things like ribcage movement etc., but in general the body stays still in Latin dancing.
There are exceptions in Smooth/Standard dancing as well. When you dance a Foxtrot or Quickstep lockstep, it is lead with hip rotation using the body's "Lazy Susan". Another exception would be a hesitation drag in Tango. The ribcage is actually bent to emphasize the dragging action. But again, in general, the body stays still and every thing happens through the legs.
As you are now aware, the way you move in Latin/Rhythm and the way you move in Smooth/Standard is very different. Keeping the body still (whichever body you are using) is very important. The still body is a critical ingredient for maintaining the partnership of the two-headed four legged animal and moving as one on the dance floor.