If you prefer the video version of the Instructable, you can see the process in this short video.
Step 1: Technique
Step 2: Wiggle the Fleshy Part of Your Thumbs Into the Stem Area
If you put your hands in this position without the apple and pull your fingers toward your wrists, you will notice that your thumbs roll out. This rolling is the force that will split the apple open.
Step 3: Grip and Roll
Step 4: Some Things I Have Learned
1. Pretty much anyone can do this. I learned it from my mom and even my 12-year-old boy scouts can do it - even the skinny ones.
2. It sometimes helps to use your knee to strengthen your grip. Try pushing on your knee with you fingers as shown in the photo.
3. It sometimes helps to start out with a softer apple or a very crisp ripe one. If the apple is not ripe, it might be more difficult.
4. Size matters. You will eventually be able to crack all sizes, but it may be easier to start with one that fits comforably in your hand - not to big, not too small.
5. If you haven't already watched the video, then I recomend you do so. Sometimes its just easier to see done.