Nagyvary has made different claims about the "secret of Stradivari" over the last thirty? forty? years, and they're all authoritative, and none of them explain why Stradivari's instruments are so valued. I'm confident that the real secret of Stradivari is a lot of things, all working together, that he learned by being an excellent craftsman with a good ear and analytical mind, and by being trained by the greatest violin maker before Stradivari, Nicolo Amati, who was likewise trained by previous great makers of his own family.

There's no one change that, applied indiscriminately to an instrument, will reliably improve it. There are lots of things that work together to make a great instrument, and a great maker knows how to balance them.

(Side note: I'm also confident that there are instruments being made today that will equal Stradivari's given time enough. In my own experience, one viola made by Michael Darnton of Chicago was markedly better than any of the old Italian instruments I've played, though they were not of the first rank. That was an instrument fit for any use, and if I could have afforded it, I would never have let it go.)