Vladimir Horowitz: How to achieve accuracy in sounding and avoid harsh tone
For accuracy, constant care is required. I always practice distinctly, never indistinctly. Then, if I do play falsely, I hear the incorrect note at once. But absolutely distinct practice is an essential for accuracy.
In my rapid passage work (runs), I play very much of the time half staccato, portato, so that every tone is very clear. This I find to be effective, even necessary, in a large hall. If I play as is called by some players legato, by others, superlegato (legatissimo), the effect will not be clear. The release of the note must be accurate, perfect, or the tones will be blurred, especially in a large hall. Therefore for rapid runs I prefer the portato, in which one note is practically connected with the next, but not held over beyond the beginning of that note.
I do not use a stroke in playing, either in runs or in chords. In rapid runs there is no time for a stroke. If you wish to strike something, your arm must be at a distance from it. You cannot strike when your hand or arm is close to the object. So in rapid playing there is no time to lift high and strike. The fingers must remain near to the keys.
The tone made by striking is not agreeable to me. I prefer the tone which is felt by the finger, and it is impossible to "feel" the note and also to strike it. In playing forte
passages, I am aware of a slight movement in the hips, in addition to the finger movements already described. This movement in the hips brings the body to the assistance of the fingers. But - the body is always assisting when the playing has life, vitality.
(From the V. Horowitz interview for the ETUDE, 1932)