It might help to think about how deep to make the crack, or in other terms, how far to push the edge through the egg. Give the egg a nice, quick rap in the middle, and have the crack go just deep enough that you can complete the split with your fingers. I think gravity should also help for this last part too.
Do it fast and accurate enough and there shouldn't be any shell pieces falling out. Too slow or too shallow and the shell won't split cleanly and will crumble, dropping shell pieces. Too deep, and obviously the egg blows up in your hands. You need speed to make this happen, but all I think about is how deep (or if you prefer, when to pull the strike back). Theoretically, even if I hit it pretty fast, the egg will only crack as far as the gash I give it.
I was taught by a chef a long time ago that the best to crack an egg was actually on a flat surface - cracking it by using an edge would always end up with some fragments of the shell in the resulting exit. That and you don't risk breaking the yolk inside if you crack it against a flat surface, although this may not be an issue depending what you want to do with the egg (Yolk splitting/scrambled eggs/making a cake or whatever).
It takes quite a bit of practice though, although I have been caught dual cracking eggs when I'm really in a hurry (not perfectly cleanly though due to being one hand dominant) and it's kinda tricky to be using a specific finger to part the shell, although apparently when you do it 12 eggs in a row perfectly without making a mess or skipping a beat, you look like a pro chef and I was told that they wished they filmed me doing it. (I've only ever pulled it off once, and I imagine I'll never do it again. That said, I'll probably not cook breakfast for 8 people in a hurry anytime soon again either.)
Bear in mind though that making a mistake by swinging too hard is a LOT worse than doing it by edge - sometimes you'll just end up with an entire egg on the table, and you're scratching your head about how an egg got out of a shell so quickly while still holding onto a shell that you could roll out flat in one piece.
More often, you hit the egg a little too hard and you leave behind a bit of the yolk as you move it to the pan.
Hitting not close enough results in a lot of problems too (Basically, if you don't swing quite hard enough, your second attempt WILL cause the entire egg on the table scenario), so it's not a novice trick - probably why the only people I know who know enough about the method to criticize my technique are chefs by trade.
