I know I sound like a broken record, harping yet again on the use of the thumb. (In the olden days, the days of my youth, a record was a disc made of PVC onto which sound was etched, and if it should be scratched, it would repeat itself.) But understanding how the thumb contributes to a fluent technique lies at the heart of our piano study. If I had to choose one aspect of piano playing to single out as the most important and most misunderstood, it would be how the thumb plays when asked to change hand location. This is often referred to as a thumb crossing, which is, sadly, misleading. Simply put, the thumb has two jobs when "crossing"—playing its note and propelling the hand rotationally to the new position.
Have you ever heard young pianists attempt to play an even scale only to be thwarted by the crashing of a careless thumb into its note and causing a bump? The sound conjures the image of a wheel with a bulge on it—thumpety, thumpety. This is often a consequence of trying to tuck the thumb under the hand, thus pinning it to the palm and making it difficult to play without falling on it.
The thumb plays its note rotationally, the result of which throws the hand to the new position. These are of course tiny movements that in speed are virtually undetectable. (See above under "demos" for more about forearm rotation. Thumb crossing begins at about 4:35.) When practicing this movement very slowly, exaggerate the rotation. When descending in the right hand, rotate slightly past the desired note in order to turn back to it. Ultimately, there is in speed a gently undulating shape. Shaping is discussed elsewhere in these pages.
This all sounds more complicated than it is. Do have a look at the demo.
