It is well known that the notes of the piano decay in intensity, losing resonance. The impossibility of sustaining a sound with a crescendo is in fact a limitation of the instrument. And it is precisely this limitation that makes the making of the legato so difficult.
Chopin's Etude op. 25 N. 7 begins with a melodic line entrusted to the left hand, apparently simple because it lacks other notes to balance soundly. Due to the decadence of the sound, we risk giving unwanted accents to these long notes, breaking the sense of the legato. Proper dynamics and natural rubato can create an acceptable sonic result, but they are not enough to achieve high artistic quality.
The higher quality legato is obtained by "spinning" the sound, in the sense of always listening to what happens after the string has been struck, always listening to how the resonance continues.
Besides, in the crescendo it is necessary to avoid giving too direct a percussion, downwards, which would create an accent that is perceived by the listener. It is necessary to try to gradually shift the weight from one note to another and with an always soft touch, avoiding a too direct attack.
To cushion the percussion of the hammer against the string, it is very useful to cushion the lowering of the key by compensating movements such as the raising of the elbow or wrist, the tangential or lateral lowering of the key. Movements that allow you to control, slow down and smooth the percussion through dragging and sliding actions.
Continuous differentiation of dynamics is also very important in these cases. It must of course be based on the indications left by the author in the score, but above all on how the previous sound "arrives". In practice, if one note is played loud, the next can be even more so as the two sounds are easily connected.
At bar N. 1 of this Etude the D sharp is a long note followed by a much shorter C sharp; due to the sound decay of the first note, the C sharp must be performed with a much lower dynamics and enter the resonance, in the resonance tail. A too strong C sharp would not have allowed the realization of the high quality legato that instead can be produced if the note is combined with the terminal resonance of the one that precedes it. In the short notes that follow (C sharp, B sharp, E), on the other hand, it is easier to pick up a higher dynamics, then play louder, as the problem of involuntary accent is less perceptible. A quality crescendo will always be easier with fast notes than with long notes.
Since the real legato does not exist on the piano, we must talk about psychoacoustics, that is, how the legato is perceived by the listener. In fact, long notes will always tend to decrease in intensity and therefore there will always be peaks on the percussion of those notes. However, these peaks must be shaped in such a way as not to create the perception of the accent, worrying about how what we are playing arrives to the listener.
To facilitate the perception of the legato by the listener, it is really important to manage the legato between the first two notes with great attention, paying particular attention to the sound emission of the first, second and third notes. This is why the legato of the second note is important even when it is louder, in crescendo, but it must always be managed in such a way as to create maximum dynamic uniformity with the first one, always listening to its resonance.