It can take a few hours to a day or two for the blood vessels to shrink down. While it is in the egg, the chick's blood is flowing through the vessels in the membrane, it is attached to the membrane by an umbilical cord, the same way a human or mammal baby is attached to the placenta. So if you tear into those blood vessels and cause a lot of bleeding, the chick can bleed to death. A little bleeding won't kill the chick, but can make it weak from blood loss, they don't have very much blood at that size! What I do is every few hours moisten the membrane to check the veins, and if they are still big and red, close the incubator and leave things alone. It might help protect the other eggs if you open it only long enough to carefully take that egg out and close it right away, and handle that egg (briefly so you don't chill it) outside the incubator, then put it back in when you're done. That way you don't drop the humidity enough to shrink wrap all the chicks--in which case, you'd have to help them all out. It's possible you've already done that, so if any chick pips the egg shell but doesn't make much progress within 48 hours, you'll have to decide to help or just "let nature take its course".