There are eight slides all-in-all but pictures can't show movement obviously so you may want to go back to the video after flipping through.
1. Frei is set up on the front post. He doesn't have the luxury to check his shoulder because if he looks away, he could miss something (a pass or shot). By the time he would look back, the situation could be completely different. RSL fans may have noticed how far off Nick Rimando is from his front post in these situations. Rimando will literally be about a yard or two to Frei's right. There are advantages and disadvantages to both positions and depending on the goalkeeper, you can play either spot right and wrong. Frei hugs the post and readies himself.
2. Frei utilizes what is known as a crossover step, which is about exactly what it sounds like. A crossover step is like karaoke (a common warm up drill) except you don't cross your lead foot behind. You keep your shoulders and hips as square as possible while repeating the step. Frei doesn't shuffle because the crossover step is faster but, as we'll see, it's easier to get moving too fast. The crossover step is the correct move here except Frei should probably drop his right foot first instead of pivoting on it, swinging his entire left side of his body like a revolving door. If he drops his right foot he can turn faster, cover more of the goal, and see the rest of the box quicker. It's not really an issue here but it's not as efficient as it could be. The ball is about a third of the way to Altidore and Frei has not yet moved his right foot. In some cases, this could be problematic.
3. Altidore heads the ball and even though Frei is centered, he is actually still moving. The downside of the crossover step is coming into light now because he has only moved a few yards and is still having trouble stopping. Frei did a good job of identifying the striker immediately from the cross but his timing is a little off. Ideally, he would be set as or very soon after Altidore connects with the ball. If his momentum is pulling him away from the ball, it's lost time for Frei.
4. Frei's momentum keeps him going to his right, pushing him off-center. It's hard to say if this was intentional or not. Hypothetically, Frei could want to cover the back post because it's easier to head the ball with momentum than against. However, Altidore has time to get behind it and sending it the other way goes against the percentage play but clearly Frei doesn't know that until right before it happens. It looks like Frei's momentum has just carried him a little farther than he wished but either way it's a small error, much like the footwork in the second picture.
The real issue isn't the angle but the timing. If you have great timing, you can get away with bad angles. If you have bad timing, angles won't matter much. The ball has travelled a good distance off Altidore's head and Frei is just now changing his momentum to move towards the ball. This is lost time for Frei and makes the save more difficult.
Something on a smaller note, Frei's right foot is planted while his left foot is arched up. Another downside with the crossover step is that a goalkeeper's feet aren't in sync. With a shuffle, the feet are landing at the same time, making a goalkeeper balanced. With a crossover step, a goalkeeper is exchanging power from one leg to another on each step. Goalkeepers have to mindfully coordinate their feet to minimize wasted time when crossing the goal face.
5. Frei heads back to the front post. It's a little blurry but this is just showing that Frei is again using the crossover step. This is another correct decision by Frei. He needs to cover ground fast and this is the way to do it. Notice how Frei's left foot (the one closer to the goal line) has never touched its heel to the ground, or even come close. It's basically just his toes on the ground here. Imagine trying to start a sprint with just your toes on the ground. It's tough to get leverage to really propel yourself forward and Frei is about to be in a bind with his footwork.
6. It's hard to tell how much of Frei's right heel is on the ground but contrast the footwork. His heel is probably not completely planted but it's easy to tell the difference in steps here. Frei hasn't completely planted his left foot but the foot shape is similar to this picture when he finally does. (The video doesn't give a very clear picture so just take my word here.) There seems to be some real indecision on Frei's part and the footwork is the give away here. If Frei thought he had more time, he would have not planted his foot so awkwardly and just played it like he's done a million times. If he thought he didn't have time, he probably would have used an overhand save (in this case, his right hand coming across) to jump the save early. It seems like he thinks he's going to have to more time than he does. Perhaps part of this is how hard it is for goalkeepers to read a ball coming at them from a downward angle, much like a baseball player trying to make a Willie Mays overhead catch. Understanding that they are very different situations, you can still see how difficult it is for a goalkeeper to handle a ball coming at him from the sky.