Once you can feel an opponent's rhythm, and have a range of your own to play with, you can use it to your advantage. You can jam them, bait them and generally screw with their timing. You can tempt them with openings and take them away, you can overwhelm them with a sudden change in rhythm, or force mistakes or rash moves.
Sonny has been called a 'ghost' on many occasions because he had an uncanny ability to appear to be in one place and then not, or suddenly close the range seemingly out of nowhere. He could also dissuade his opponent from attacking by timing their intent, or lead them into freezing or panicking by doing something unexpected.
The best analogy I have is a musical one - he could bend notes, or subtly extend or shorten them to tweek the tempo and steal range or force his opponent commit. He could syncopate, change tempo suddenly or slowly, turn up the volume or become barely audible.
He never explained what he was doing with musical terminology, but as he was a musician and composer, and also a dancer, it seems fitting. Music was a very important part of his training sessions - we rarely flowed without something playing in the background, and I know for sure that he was very precise about the pieces of music that he chose to play during flow, choosing them as subliminal indicators of the tempo he was trying to get us to feel.
Of course I did not realize this for many years - only after I started to notice the part that rhythm played in dueling.