Dear Sammy, to facilitate the various thumb passages in scales, Chopin used to start from the key of B major for the right hand, and D-flat major for the left. At first, he would have the hand kept calm, making the notes played in such a way that the sounds were very legato; then he would have them played staccato without worrying much about the thumb passage. One exercise, for example, was as follows:
Right hand:
1st finger on E, 2nd on F#, 3rd on G#, 4th on A#, turn/thumb under to 1st finger on B, 2nd on C#, 3rd on D#, etc.....
Left hand:
4th finger on Gb, 3rd on Ab, 2nd on Bb, 1st on C, turn/thumb under, 3rd finger on Db, 2nd on Eb, 1st on F, etc....
Then one would move on to more difficult scales.
Since every scale has fundamental positions, by playing first legato and then staccato, and then with various accents on the 1st, 3rd, and 4th fingers, the hand acquired familiarity with the various successions of notes to the point of achieving a good thumb passage through muscular action without causing the hand to lose its correct horizontal position. Many of Chopin's students asserted that through this technique one arrived at playing legato very well and performing very difficult and particular passages, typical and revolutionary in Chopin.... such as, for example, passing the thumb beyond the 4th and 5th fingers. Despite the numerous novelties and originalities regarding fingering, Chopin still referred back to the Clementi school. I think it is nevertheless necessary and fundamental to be followed by a good piano teacher, especially regarding the correct position of the hand in the various thumb passages, because talking is one thing and doing is another, and a teacher beside you will help you not to acquire wrong positions. As for the thumb, I know nothing more. bye.