[mylife]

placing sand on the soundboard and running a bow along the edge of the soundboard will set up a vibration that will reaveal a specific pattern in the sand.

He may have also used this technique to achieve a particular resonance

[RightWhale]

It will do that. Another factor is the location of the bass bar relative to the sound post.

I wonder how Stradivarius got started. Who would have fed him while he was figuring out his technique; the time involved must have been extensive and he wouldn't have had time to hold down a regular job or even maintain a garden.