look at horowitz’s slowmo vid of 10/8 on the art of piano, or on tube somewhere. imo that’s the most perfect example of thumb under and thumb over mixed, the way it should be. there’s too much discussion about thumb under or thumb over, it’s both.
practice slowly as legato as possible, sticking 3rd or 4th finger on they key while your thumb passes under and plays the next key, then prepare 23(4) on the following notes. preparing is very important. helps you relax as well. check cortot’s technique book for some nice examples. lechetizsky has a whole chapter about this too in marie pretner’s book.
think in groups, but practice on each finger seperately in your head as well.
it’s all in the brain, that’s probably the most important. relaxation is about making yourself relaxed by thinking it and making your fingers do it through your brain. too many pianists that prax technique try to do it all from the fingers and from playing. it’s all about the brain and ears dude.
also prax scales and arps with 121212, 13131, 14141, 151515 etc. and while youre at it play scales with busoni fingerings too, look up his klavierubung somewhere. (stuff like 12345 23 12345 23 and 12345 12345 12345) these fingerings completely exclude ‘weak fingers’ and force you to treat every finger as equal. it will have a great effect on “orthodox” fingerings.
dohnanyi has arpeggios with 1 on every underpass, even on black keys. it’s a really good exercise to do, but of course you hardly ever use this fingering in actual playing, but it’s good to relax the wrist and learn to pass under onto black keys with your thumb, and 4 over 1 from black to white.
from the mendelssohn vid your hand looks good actually, just a bit stiff. try to do isolated finger prax ( like putting 1234 on their keys and then doing all sort of rhythms on 1 key with 5 while keeping the hand as relaxed as possible). these exercices used to be for real finger indepence, which has been scientifically proven to be wrong, but they still have a lot of use in playing.
i’ve also seen that your elbows are really close to your body. this is the kind of tehcnique harpsichordists and early pianofortists used. in the 19th century arm tehcnique was a lot more important. try to relax your shoulder more and let your elbow lead your hand. like if your ascending with your right hand, try to move your elbow to the right and sort of lead your hand. this helps in relaxation as your hand doesn’t need to do it all anymore. this is most helpful in arpeggios.
if you need to play quick apreggios, it’s all in the elbow. this is when you really use thumb over only, and this is only possible to do smooth if you let your arm (elbow) help in this.
don’t try to think too much about your arm movement. just don’t let your fingers do all the work, this gives a loud but sort of thin tone. more arm weight helps in tone.
play some more pieces that require a lot of wrist rotation. or just prax big broken chords like C E G C E with 12345. do this symmetrically inversed with your left hand (that would be on E C A E C) and try to use as much wrist rotation as possible with the most possible relaxation. this way you just teach yourself to relax, even in scale passages. or like the D A D A arpeggios in ravel’s jeux d’eau.
sorry about my english but if i type a lot i don’t try that hard. might be a lot of bullshit, i can’t be bothered to concentrate too hard, but feel free to ask some more, i’m really into this stuff.