That “float test” is a method you can use to tell if there are live chicks in the egg. It’s customarily used when the hatch appears to be over or running late and you have unhatched eggs you want to check. If the chick is pretty well developed and you put it in a pan of still water, if a chick is still alive in there, the egg will wiggle on its own. As long as the egg has not pipped, it won’t harm the chick. If the egg has pipped, you can drown the chick.
It does not tell you which ones will hatch. It tells you which ones still have live chicks in them. Not all live chicks make it out.
I don’t know at what stage of chick development the float test works. I do know the eggs can be a couple of days early or a few days late, either in an incubator or under a broody.
To me, its benefit is determining if the hatch is really over. I just candle the eggs when I go into lockdown, toss the clears, and give the rest a chance to hatch. I doubt that float test will hurt them as long as they are developed enough to actually wiggle so you don't toss good chciks and they have not pipped, but I don’t see how it would help them to hatch in any way. They are yours and you can do as you will, but I don’t see any reason to go through another step with them. I find the less I interfere the less damage I do.
