Noctave
The piece
The Fantaisie-Impromptu, Op. 66, composed in 1834, was only published after Chopin's death, in 1855. Its fast section, marked Allegro agitato, sets two divisions of the beat against each other: triplets in the left hand, sixteenths in fours in the right. This exercise, written for Noctave, is not the piece itself: it draws on bars 3 to 6 and brings the right hand in by stages, at a Lento tempo.
Two bars of left hand alone set up the triplets, C-sharp, G-sharp, C-sharp, E. The right hand then enters with one note per group of three, then two, then four, taking you from one note per beat to two against three and then four against three. In the last stage only the first notes of each beat coincide; the right hand's second note arrives before the left's second, its fourth just after the left's third. Keep the left hand perfectly steady and let the right slot into it, minding the F-double-sharp in its figure. Loop the exercise until the hands stop hunting for each other.