Mozart, Sonata in Bb major, K. 333, mm. 57-59.
In Alberti bass (left hand, above) the notes continually change directions, and it is possible to alternate supinating and pronating movements to play this type of pattern at the piano. When the downward motion of one side of the forearm engenders the raising of the other (as with a see-saw with a preposterously long axis of rotation), so as to lift the fingers in the desired succession, you have “single rotation.” “Single rotation” is not limited to situations where the notes move in opposite directions: this is just the easiest-to-understand example for using this coordination.
As you might imagine, you can play (as few as) two notes with one hand using a single supination-pronation coordination, or pronation-supination coordination, for single rotation to be operative. In fact, any two notes (but not any three notes, as we’ll see in a moment) can conceivably be played using such a coordination.
To say it again:
Single rotation requires that supination and pronation alternate for the purpose of lifting fingers so that they may descend into target keys. No fewer than, but at least, two consecutive notes can be played using this particular coordination.
The single requires a coordination not unlike that used to jiggle a doorknob, or by the dancer for “jazz hands.” To get the basic gist of the kinds of situations suited to single rotation, pretend-play some Alberti bass while seated at a table using only forearm rotation. (You can try it at the piano, too, but having to press keys does complicate this coordination.) Single rotations are also used in many less intuitively obvious situations, and I will get to that.
Now, before I tell you about the arguably more cryptic double rotation (and by the end of this article I will be explaining that it needn’t be hard to understand this aspect of Taubman’s organizational scheme, on an intuitive level) you must understand something VERY IMPORTANT!--about my use of terminology.
If I’ve supinated my arm to raise my thumb, obviously my forearm will next need to pronate to bring the thumb down into the key. The supination is the upstroke that paves the way for the pronation, which is the downstroke. This will be perfectly obvious if you will try it.
In a technical approach that is a remnant of days of yore, pianists learn to lift fingers individually to play. Functionally, forearm rotation replaces this biomechanically naive idea that has created so much pain and disappointment for so many pianists. You prepare your thumb for playing not by lifting it, but with a supinating motion of your forearm, before the downward swing into the key. I use “supinate” and “pronate” for the prep swings only because I assume that there will be a lot of gravity in the descent, with the muscles behaving passively to the greatest possible degree allowed by one’s sense of what one is playing and its context. It is possible to use the terms “supinate” and “pronate” for only the downward swing into the key, too--but then you would also have to account for the mechanically more energy-intensive part of how the fingers manage to get back up, over and bloody over, sometimes at blinding speeds.