The secret of the key weight of the Horowitz Steinway, when it was in mint condition in the care of Franz Mohr, was not only the diminished down-weight, but also an increased up-weight, both within 10 grams.
A gesture, passage, arpeggiation, run, and ornament, which is played at the appropriate speed and with dynamic contour, amounts to more than the sum of individual keystrokes, as its parts melt together to bring forth a new quality and aesthetic. Like looking at a painting from the right distance.
Comfort is not exactly what we are looking for at the piano even though we find it if we seek efficiency, elegance, being "in ones element" ~ like a bird in flight. One has to feel good and technically competent while playing. The performance should be a positive, pleasant, and rewarding experience. The feelings of fear and strain on stage are ultimately a result of adverse preparation. Practice relaxation, suppleness, clarity, and dexterity in fingers and arms; practice to merge movement and instrument, practice to become a Centaur.
The four Lumbricalis muscles in the palm are covered by the surface of the palm, glabrous skin tissue. When the 4th and 5th fingers are tucked, the 2nd and 3rd fingers have a little more power to strike the keys due to the increased surface tension in the shear-stretched skin of the palm between the 3rd and 4th fingers. Used as a tool it can very effective: observe the use of tucked outer fingers in the hands of Vladimir Horowitz. He tucks like a crab and stings like a scorpion. It works only in combination with a flat finger technique, which Horowitz was a master of. My father as well as Leon Fleisher mentioned the flat finger technique to me as an arcane and almost lost art.
The art of the flat fingers is an arcane but not lost art. Important is to play the black keys deep in the keypad so the thumb is relaxed on the whites, and let the thumb move under and between extended fingers and off the keyboard when necessary. The hand is almost flat, the knuckle-ridge is relaxed, and the wrist is on the level of the fingertips. The flat finger is a straight extension of the entire key, touching down almost at the joint with the entire first phalange finger pillow on the key, gripping gently.
A straight line from the elbow through the forearm, through the wrist, through the knuckle ridge to the fingertip and further straight through the keypad, the pivot, the entire key all the way to the jack tip propelling the hammer. The connection through the single key is to the entire key bed in slow and melodic playing, and to the surface of the key in fast and ornamental passages. The up-weight of the key lifts the relaxed fingers after action, no need to yank or curl. The impediment of the fourth finger to lift above the neighboring fingers becomes obsolete. Curvature (as little as necessary) of the fingers serves to provide for the most comfortable position for the group.
The flat finger can play slow and melodic line with exquisite control and expressive singing quality, bel canto; and fast and light passages with sublime transparency and the quality of falling snow, jeu perlé.
Sometimes one has to play very fast octave passages not in a melodic but in a mechanical way, very short and percussive, with a very fast, machine gun like, repetition push (Stoß) of the arm. The hand and forearm drop on and bounce off the keyboard using the momentum of arm weight and elasticity of tendons and muscles.
The wrist is in most cases not the engine, the initiator, or a mainspring of movement and momentum ~ but functions as the suspension, gear, and transmitter of the movement and momentum generated by the weight of the arm.
One has to in effect develop three kinds of piano technique: a Classical (simple clusters, Art song, orchestral transparency), a Romantic (complex clusters, Bel Canto singing, orchestral might), and a Modern (fists, elbows, paper clips). One builds on the other. When the techniques were established and developed, they seemed to replace the previous ones, but only for the ignorant observer: They augmented each other. Today we need to acquire all techniques in order to be able to play repertoire ranging from Couperin to Cage. By the time we accomplish this, we will have cultivated our own technical and stylistic competencies and preferences.
There is an absolute, unlimited, infinite, enlightened piano technique. It is efficient, comfortable, elegant, ergonomic, aesthetic, natural, and personal. Like the movement of fish in water. Seek that with all your heart.
For ultimate virtuosity: gain in lightness! Throw away the armor and heavy weaponry - speed is weightless precision!