I don't know alot about breeding, but I think the idea behind finding breeding stock is simple. You grow out as many plants as possible, and select the ones with certain traits you are looking for. Example, a male perhaps that stays very short, and maybe has alot of resin on the leaves, or what ever traits you want to try to pass on to the next generation. Often times when you grow out a large number of less stable genetics, you encounter mutations, or very rare phenotypes. Depending on what things occur, you may want to save those genetics, and use them. Natmoon had a thread about this crazy mutant pheno he came across. It was SUPER resinie, but it didn't hardly produce any buds, it had strange spikey bud leaves. I think he bred it with one of his other strains to try to pass along the redicules amount of resin the plant produced. Any way, like I said I'm not a breeder yet, but thats my basic understanding of it. Each time you cross the plants and then grow them out, you try to pic out specific traits (smell, growth pattern, speed, high, taste, yield) and then cross the plants back to increase those traits, and stablize the genes. Hope this helped, Peace TC.
NOTE: If any of this information is incorrect, and a breeder wants to correct it, I am eager to learn more on the subject!