They pip externally with full blood vessels but they rarely start to zip. If they did start zipping and still had full blood vessels you would see bleeding during the zip and when they hatch with the yolk sac not being completely absorbed it's usually due to another set of issues, too high of temperature, too high of humidity during incubation, and sometimes infection. I've hatched hundreds of chicks and those that have known me, know that I'm not very hands off, lol, so I promise I have plenty of experience in these things. It's a very low likelihood of stepping in too early once the chicks are zipping. But as I mentioned, going in through the air cell is a good precaution just to be safe when you're unsure. Hope I explained that better...