A lack of oxygen is what causes them to start twitching and pip through the shell. The problem with doing this for them is that you'll most likely have to help them through the rest of the hatching process and you don't know if they've finished absorbing the blood and yolk sac yet. My chicks always take their sweet time from internal pip to external, then to zip and hatch. If your settings are & have been right and the chicks are healthy, they should hatch out all on their own. Hope you get a great hatch!