How do you breed an auto flowering strain from a non auto flowering strain?

I'm interested in breeding, but I don't understand how the breeders select plants that autoflower.

Mr. Diamond
I love Seeds!

they pick the ones with the ruderalis in it.

Many different ways most common which i dont really like is
auto x photo then back crossed to auto then line breed until stable but you only get a few % extra photo genes in there