This is the advice I was given in the other thread... because they usually can't break out on their own.
"Chick must make its own first crack in its shell, not a full pip, but a crack. If the other eggs are pipping and cracking around the shell, it is time tostart the rescue of the "breach baby". Toothpick break into the membrane near the beak. Air! Then,if no bleeding, chip a bit around, but higher than the usual "top"--air must get to the beak area. Chick cannot turn, so if all is well when the top comes off, gentle chipping down the egg, will begin to release the body. Let the baby rest before removing the entire shell. Almost all breach babies have thrived==would have perished without carefully times help. GOOD LUCK"
Assisted hatching is controversial, but I am not sure why if you have a small flock or rare breeds or just don't manage incubation quite as optimally as it should be. You're just taking a chance at improving the odds for some that may not survive. If you are careful, you won't accidentally kill too many by helping too early. In over 120 eggs set this year, I have had only one "breech". I don't think it is as common as often thought, when the air cell expands it may appear that a pip is too far down, but it may be perfectly normal.
If I feel a chick may be struggling, I look at the following:
How long has it been since the shell chipped? Has the chick pipped out to fresh air? If more than 8 hours has passed and no external pip, I candle and check for movement or chirping. If there is movement or chirping, I chip off a piece of shell at the chip, and open a small hole for the beak, then put the egg back. If there isn't movement or chirping, it may be too early, too late, or the chick may be resting. I put my ear up to the egg and listen for breathing (sometimes a slight clicking sound), and if I hear breathing I put it back and restore the humidity.
After there has been an external pip for more than 12 hours, I watch for zip and listen for chirps and watch for wiggling eggs. If zipping is going on, I stay hands off. If there is a pip, with or without chirping, but no sign of hatching after 24 hours, I usually assist. I am hatching out turkeys, and they seem to be growing too big for their shells in the last couple of days as quite often they have no room to turn. I expand the hole IN THE SHELL ONLY around the pip toward the air cell. It's okay to open the air cell if the chick has been breathing outside air for a day. Be very careful not to damage the membrane until the air cell area is exposed, then gently and carefully tear open the shell membrane only so that the inner membrane over the chick's head and wing can be seen. If you see blood vessels or a red/pink tinge, STOP. Wrap the egg in a moist cloth, leaving a hole for the beak, and put it back in the incubator. You've gone in too early, and the chick is at risk of bleeding, breaking the yolk, or getting an infection or having navel complications. Don't encourage it to hatch yet; when it's ready, you've made the exit path a little easier. Watch for the struggle and remove the cloth when the chick is ready.
If the inner membrane is clear and you see the natural color of the chick's down, gently ease the membrane off the beak and face. Often a chick will feel the removal of the restraint of the membrane and begin to kick and push and muscle its way out. Let the chick do the work. If you don't leave sharp, unbroken shell around the air cell, the rest of the hatch should go fine. Give the chick an hour or so to push free of the shell; by then the yolk should be fully absorbed and the umbilicus ready to part. If the chick won't push it's way out and seems weak and lethargic, chances are it has an issue and may not survive. Your call on whether to "deliver" and possibly cull later, or just don't help any more.
You can't save all chicks, and you may kill a few by accident. But you may end up saving some that otherwise would not have survived. Ideally, though, your incubation has been managed so that you don't need to or want to interfere, and you have baby chicks popping out right on schedule with no effort at all!