While I would love to do that one-handed egg-cracking trick, my cooking style is a touch messier. And my sometime assistant, age 6, is not much neater about it. Thus we eagerly tried this trick, first seen by us on a Jamie Oliver (sigh...) show.
To remove a broken piece of eggshell from your cracked eggs, fish it out with another piece of eggshell. They will find each other and it's so much easier than going in there with slippery fingers.