This is from the articles section....
Another more advanced variation of the curveball is the knuckle curveball (sometimes called a spike curve). This is the curveball grip that I used. Thrown the same way as my beginners curveball only you'll tuck your finger back into the seam of the ball. Your knuckle will now point to your target instead of your index finger (in the beginners curve).
The difficulty with this pitch isn't from the pitch itself. In fact, most pitchers feel this grip gives them the most rotation — and most movement — of any breaking pitch. However, many pitchers who are learning this pitch for the first time, aren't comfortable with the "tucking" part. It's not super comfortable at first to tuck your index finger into the baseball.
This is why I recommend that you spend a few weeks — preferably during the off-season — working on tucking your index finger into the baseball. Do it while you're watching TV or in study hall at school. Once your index finger is comfortable with the grip, you can progress into spinning a baseball to a partner without any trouble.
Note: You've got to maintain short and well-manicured nails — especially on your index finger of the throwing hand — for this pitch to be effective because long fingernails can get in the way of the grip.
One thing you can do is apply a thin coat of nail polish or fingernail strengthener. It's in the women's section where fingernail polish is found, of course. It's shiny (even the matte finish is a bit shiny), but dries clear. And it helps to make fingernails a little tougher. (If you do use it, you really need just apply it to your index finger.)