There are things explained about the movie, such as the fact that Forrest only believes in three things God, his Momma, and Jenny, and everything else in the world that he experiences has to be filtered through those three things. It explains how the movie is not about anything that happens in American history that we see in the movie (from the racism that he encounters in his early college days to the Vietnam war). There's no bad guy, there's no overarching quest, it's simply about the spirit of Forrest Gump.
There is also some excellent coverage of the filming of specific scenes, such as the Dr. Pepper scene (which had Tom hanks drinking a massive amount of Dr. Pepper it reminded me of how Charlie Chaplin ate so many licorice boots in the famous scene in The Gold Rush that he had to have his stomach pumped), as well as the scene where Forrest shakes John F. Kennedy's hand. I loved seeing how they managed to do that, although I wish they would have expanded it a little, at least enough to show how they managed to get JFK to say, "I believe he said he has to go pee!" I tend to doubt that they actually found historical stock footage of Kennedy saying that!
Another thing that I felt should have been in this documentary was how they managed to portray Gary Sinise without legs, because it is one of the more memorable digital effects that were achieved for the film, although this is explained elsewhere on the DVD, and is probably why it was not included in this documentary, which I believe was made after the release of the original DVD. Much time is spent on how they did the feather at the beginning of the movie, although that is pretty important since the feather embodies the theme of the entire film.
There is some great explanations of characterization in the movie, like how Forrest and Bubba became close friends because Forrest was the only person who would listen to Bubba talking endlessly about shrimp. Forrest accepted Bubba unconditionally, just like Jenny did for him.
Also interesting is the commentary made about Vietnam being in the movie. Just like the theme of the movie being about Forrest and not about the world in which he lives, there is no comment made about Vietnam, how bad it was, etc, it just is. It's simply another experience that Forrest goes through, and it is there to enhance what we know about Forrest, not to make any comment about the meaning of the war itself. This is also why it is shown only through the eyes of Forrest Gump, we see only what he sees. That this theme permeates the rest of the movie is not surprising. It may have had something to do with the title of this documentary.
There were other interesting things shown about making the movie, such as the fact that they did the storm scene late in the film using an actual jet engine, which I don't think had ever been done on a movie set before. But they used a real boat in real water and had people on each side pulling on huge poles that stuck off the sides of the boat like wings so that it would rock realistically, while water cannons and a jet engine were trained on it to create the effect of a terrible storm. So while this documentary doesn't explain how a few things were done, it does explain a tremendous amount of things, and delivers what it promises. Very good.