Rapping the back doesn't have anything to do with string ratios/bridge placement... just has to do with checking whether or not there are any gaps or openings in the wood. The distance between the soundpost and the bridge makes huge differences in sound. Even a slightly tilted bridge makes a whole violin sound like junk, and if you're not careful, it can snap back and have the soundpost come loose, and then all of a sudden your violin doesn't look so good anymore.
I did use to play the strings on the other side of the bridge for fun, but don't really get the joke. Speaking of Lalo, I'm going to the Kennedy Center on Saturday to listen to Josh Bell play it. Not particularly excited, but live performances are nice to attend.