Thanks to everyone for responses, information, and suggestions.
Thanks Chuck for the Campy catalog reference, and I look forward to reading it.
In practice the shifter does permit a range of techniques (part of its unique charm?). It does seem possible or even likely that people would have been motivated to figure out their own tricks or creative approaches. For example, creating chain slack with the shift lever sounds like an interesting maneuver, given that with the same hand, at the same time, you would want or need to be closing the QR lever slightly, whether to keep some drag on the wheel so it doesn't go too far forward, or to maintain the new axle position without sliding backwards again.
And shifting while climbing, which requires both backpedaling (to shift) and an immobile chain (to create slack) according to the official instructions, does seem like it would strongly inspire alternative approaches!
It sounds like from what you write Steven that your experienced friend didn't teach you the official method for creating chain slack (shifter pushing laterally on immobile chain).
I'd like to know what method Bartali actually used to shift--did he use the official method or his own?
In the campy catalog picture of Bartali climbing and shifting, the only thing that's clear is that his right hand is next to the levers, with fingers flat and fully extended. This seems like a common sense way, as you put your hand down to shift, to avoid missing the levers and putting fingers or knuckles into the spokes!