What is core shift?

One thing to keep in mind when choosing a block regarding core shift. The closer to the end of a run with a series of engines, it is more likely to have core shift.....at least this is what I have always read and heard. True or not, I don't know. In other words, the molds are newer towards the beginning run of a given engine. Say the 72 and 73 340s for example would have more core shift than the early ones....at least according to that theory. Whether there's truth to it or not who knows? Just something I've always heard. I guess it all depends on how many blocks were cast off the same mold and how often the molds were replaced. I'd like to pick someone's brain that actually worked in the foundrys and find out. Best thing to do if you're in doubt is sonic test it.