Inside of the foot. In Figures 1 & 2, Jeter and Vizquel both are stepping in front with the inside part of their right foot pointing towards first. The reason is the same as why a pitcher in the wind-up turns his foot sideways on the pitching rubber before lifting his knee up to throw. It enables him to turn properly before opening up to throw.
Turn the shoulder. Shortstops turn their front shoulder in and point it to their target as they are crossing in front. If they didn't, they would fly open too soon (similar to a pitcher) and lose velocity and accuracy.
Bigger the better. The four-step footwork process should involve four large steps. Smaller steps limit balance, momentum, and power. The third step (right foot cross in front) needs to be a big step, almost a jump. Many call this a "crow hop" which develops lots of momentum towards 1st base. A big jump/crow-hop with the third step is what allows shortstops with less arm strength to continue to play shortstop (ex. Ozzie Smith, David Eckstein, etc.). Not only does a bigger step cut the distance of your throw, it gets the most out of your body's momentum so you don't need your arm as much.
- A play up the middle where crossing over continues his momentum towards centerfield. Crossing behind with the next step after catching the ball redirects momentum towards 1st base.
- A play where the runner on 2nd base incorrectly runs to 3rd base on a routine ball to shortstop. The shortstop steps behind on that play so as to turn towards 3rd base to throw the runner out.
- A medium speed ground ball in the hole that is hit too hard to get to the right of but not hard enough to wait for it deep in the hole. The shortstop cuts in, backhands it, steps behind, and throws to 1st base.
- A play at home plate when the shortstop starts in on the grass. Stepping behind shifts his momentum towards home plate.
It's a bit technical but the great ones do it.