The guitar maker I know knocks on the raw soundboard and listens very close to the response. He can tell how it will sound in a guitar and how the guitar has to be built to support this particular soundboard and wanted characteristics of the guitar. (There are a lot of variables)
He explained the difference in the knock response, and as musicians we could hear the difference too.
It is called tap tuning and violin makers have been doing it for years. We don't know what Stradivarius, but nobody would be surprised by the claim that he was doing the same thing when making his violins.
There is however a lot of debate in the luthier community on if it actually matters. Some claim it does and spend a lot of time on it, and have even setup fancy test fixtures to show patterns with different frequencies. However some claim it makes no difference and [I'm not sure what]. Both produce great instruments.
