Well, let me share my experience. I help ALOT. I know I shouldn't. Usually when I help, there's a bit of blood, which tells me that the chick wasn't quite ready. I've never lost a chick that I've helped along, but they can be in that egg for quite some time, trying to get out.
Usually, I can tell that they hadn't absorbed their yolks enough, or that the membrane is still kind of veiny and bloody, and then I end up leaving the baby in the incubator an extra day, and it's fine.
But, truth be told, I shouldn't have meddled. On the rare occasion, I know I saved the chick, but 99 percent of the time, the chick just needed more time.
Just make sure those humidity levels are up, and try to set on those hands........
Sharon