R is very good. One thing that he's really good at doing is being able to move on the balls of his feet. What this gives him is the ability to do more things than if his foot was flat on the floor, a really cool one being he can change the angle of the heel to evade and/or counter a foot trap/leg 'bar'. His pivot and drop at the moment of contact is very effective.
Basically he SHEARS the angle (using the same point of contact but utilizing a direction that favors him) with this new angle, he can put his heel down and take your balance instead of you taking his.
Of course on can also use the heel as a pivot and connect the knee to either as a one-two hit depending on if the range opens or closes. Sonny played mostly on the balls of his feet but used both pivots.
What articulating the ball/heel and foot/knee gives you is leverage. You come to see the arcs as well as the straight lines (both away from you and towards you) which you can use for power generation and the ability to string multiple hits together.
I think I already wrote about how Sonny vary rarely used his first contact as the final hit ... I mean if a single attack line was there, sure, he'd use it, but usually he was fighting out of his weight range and needed to off balance before striking for the greatest effect.
Not being flat footed takes training because we all have a tendency to plant ourselves when we fight. I'm sure this is partly to do with a freeze response, it also feels more powerful. However, it is not so good for evasion, countering, drawing, or reversing attacks which are all great skills for smaller people, and if you are around swords.
Here's a good way to start getting light on your feet (If you are not interested in what you should wear to look good during your practice, start at 3mins 15):