If you were to superimpose the two photos above onto each other, the two red lines would draw an X. What's happening here is the goalkeeper (David De Gea - you might have heard of him) is propelling his body upward to get a hand on the ball, and then his legs are carrying on upward as his upper body pitches down, contacting the ground first.
It's important to understand that the body shape here - the X - is not in itself critical ; it's what it indicates. 'Drawing the X' indicates that the goalkeeper exploded off the ground with maximum spring and power. If your feet 'keep going' and end up over your head, momentum dictates that your body has no choice but to react in this way. Now, there's more to making this save than just the explosive spring off the ground. You need the upper-body flexibility, the reach, and the sheer will to claw the ball to safety. But having the spring is an excellent start.
Sometimes goalkeepers who most definitely have the requisite athleticism don't, for whatever reason, get the timing or the spring quite right. They start the dive by pushing off their lead leg, drawing the X's first red line - but this body shape never changes, the feet drag along the grass, weighing them down, and they fall back to the ground without ever changing their body plane, the lower body landing first. Sometimes they still manage to make the save; sometimes they don't.
Watch this Andrey Arshavin goal vs Manchester United many years ago. I've always thought Ben Foster's dive looked a little strange, and I think it's because he never really gets his lower body powering off the ground. And if the lower body drags behind, the upper body is going to struggle to extend.
In still frames below, you can see his feet dragging along the ground, and his lower body contacting the ground first when he lands. Again, this may not be the technical reason he didn't make the save - but it indicates he didn't get the greatest spring in the first place.
This is where video analysis can be so beneficial for young goalkeepers. Watch your body shape as you deal with these shots. Can you draw the X? If not, it starts with lower body speed, power, and explosiveness.