It's called pipping the eggs. Babies that don't pip on their own risk dying in the egg full-term. It can be a debatable action, as some people believe the ones that don't hatch on their own were not meant to be.
I certainly wouldn't use a razor blade though...
: Hatchlings that don't slit the egg shell after all others have hatched can be helped by making a shallow slit in the top of the shell with a razor blade.