It works well from day 1 to day 3.It's false in any line not bred for it and it has to be specifically bred for and only applies to day 1.
It's not like autosexing so it's not in a "line" of a breed. It's sex linked so you have to cross certain birds for it to work.
I've heard there's certain breeds that they do it with but for that to be true it would have to be a breed with two seperate lines maintained. One fast feathering and one slow feathering.
Doesn't seem like that's something anyone would bother with
