I felt like this part could use some fleshing out. I'm pretty sure that the egg does not lose its strength because it "loses its curvature", tough I must admit I'd be hard pressed to provide a more precise explanation. I would say it has more to do with the "hand wrap" spreading out the force over a wide area, so that the egg's curvature causes the hand's force to work against itself. But a small discontinuity creates a focus point where the force will be localized, causing successively more breakage in the shell.