From the Wiki Bio on him:
Horowitz's performing style frequently involved vast dynamic contrasts, with overwhelming double-fortissimos followed by sudden delicate pianissimos. He was able to produce an extraordinary volume of sound from the piano, without producing a harsh tone. He could elicit an exceptionally wide range of tonal color from the piano, and his taut, precise, and exciting attack was noticeable even in his renditions of technically undemanding pieces such as the Chopin Mazurkas. He is also famous for his octave technique; he could play precise passages in octaves extraordinarily fast. When asked by the pianist Tedd Joselson how he practiced octaves, Horowitz gave a demonstration and Joselson reported, "He practiced them exactly as we were all taught to do." Sergei Rachmaninoff himself commented that Horowitz played contrary to how they had been taught, yet somehow with Horowitz it worked. Music critic and biographer Harvey Sachs submitted that Horowitz may have been "the beneficiary - and perhaps also the victim - of an extraordinary central nervous system and an equally great sensitivity to tone colour". Oscar Levant, in his book, "The Memoirs of an Amnesiac", wrote that Horowitz's octaves were "brilliant, accurate and etched out like bullets." He asked Horowitz, "whether he shipped them ahead or carried them with him on tour".
Horowitz's hand position was unusual in that the palm was often below the level of the key surface. He frequently played chords with straight fingers, and the little finger of his right hand was often curled up until it needed to play a note; to Harold C. Schonberg, “it was like a strike of a cobra.” For all the aural excitement of his playing, Horowitz rarely raised his hands higher than the piano's fallboard. His body was immobile, and his face seldom reflected anything other than intense concentration. Horowitz preferred performing on Sunday afternoons, as he felt the audience would be more well rested and attentive than during a weekday evening.
Everything he did allegedly incorrectly apparently worked for him .. you can see in the video above that he barely broke a sweat in those octave runs, which are very demanding, physically.
