kieck writes:
"Hybrid vigor" refers to a cross between two different breeds or races, not to a cross between two different lines within a breed. Think of cattle breeding. The "hybrid vigor" seen in black baldies, for example, comes from crossing a black angus and a hereford. The offspring from two separate lines within the black angus breed will not have the "hybrid vigor." The reason, in a simplified explanation, is that even separate lines within a breed are still remarkably similar to one another.

tecumseh writes:
I would suggest this is not the best example since the two breeds mentioned are both from the british isle and are therefore not that distantly related. a better example (and the one most often used in animal science text) would be any english cattle breed crossed to zebu stock.

I would suspect (based on what other more knowledgeable folks have told me) that the idea of keeping the russian stock 'pure' in regards to mating has much more to do with the aggressive nature of the cross than anything else. for most beekeepers and many queen breeder how you might accomplish this simple thing would seem to me to be quite complex.