What has always intrigued me is the manner in which Horowitz could, almost by illusion - either by the use of the pedals, accentuations or using the acoustics of the hall and the instrument - make his legato sound as if it were created by non-legato, portato attacks, which in fact was according to Busoni, the basis of piano playing. (Busoni considered all legato to be an acoustic illusion.) With Horowitz it is what happens between the notes that is more important than the commencement of each tone. He once said that he was more concerned by how the notes came up than by how they went down. This, in combination with the dynamic tensions created by his use of the pedals, gives a tremendous potential for colour and contrast within passages in which the pedal was released from duty.
https://www.sfinstituteofmusic.org/horowitz_interview.pdfHe does mention about the 5th finger being the guide, I never quite know what he means.
Glenn Gould's finger tapping, which he learned from Guerrero, also places as much emphasis on the release of keys, or the 'finish', as it does the attack. https://www.musicandhealth.co.uk/articles/tapping.html
If you watch Michala Petri https://www.michalapetri.com/you'll note she keeps her left hand finger 5 fixed on the instrument at all times. (That finger is the only one that does not normally cover a hole.)This is a bit controversial and rarely if ever taught. On the other hand, she has been acknowledged as the world's premier recorder player for decades. I've seen her in person several times, and though expensive it was always worth it. So I've started playing that way, and I think it stabilizes and guides the left hand, but you do have to be wary of creating tension.
Like, do you think it creates a sense of "connection to the instrument"? (a possibility)
I read the article over a few times. He isn't thinking mechanically so if you're trying to find a mechanical explanation I don't think you'll find it.
Are you sure?.
I was talking only about what the article says. Not what he might actually do. (Clarifying).
I didn't analyze anything. I paraphrased what that passage says.
I seem to remember him always curling his 5th finger when not in use. Whatever significance that may have, I don't know...