The question was about the Rachmaninoff concerto but can it be applied to anything?
if your finger stays tense your thumb is not standing well, same for fingers, if they are solid,...
Kind of a continuation of a previous answer on practice
Excericess are extreme version but they are created to awake your reflexes up
Alan answers a question while demonstrating on Chopin's Op. 25 No. 12
Alan is answering a question and touches on Peter Fetuwanger's technique ands Chopin Op. 25 No. 2
Let the wrist flex
Using the wrist to inflect the music, to form the music
Going laterally helps the fingers and makes the angle of arm to finger to key actually good