I just rewatched the flip play, which is here:
http://m.mlb.com/video/v3134880/nyyoak- ... conic-flipThe camera doesn't follow Jeter (why would it?), but if you watch the flight of the ball and the advance of the runners, and know who is in RF and who is on first base, and keep a peripheral eye on the pitcher, Mussina, I think you can explain why Jeter was where he was, and it shows excellent anticipation on his part.
Giambi had singled with 2 outs. He's a very slow runner. So Giambi's on first. Long is up and he slices a line drive over first base that lands just fair and then kicks into the big foul territory in Oakland Coliseum. The RF is Shane Spencer, who really has a LFs arm. Spencer has a long run into the foul corner to get the erratically bouncing ball and has to turn and make a tough throw to home to have a chance to nail Giambi at the plate. The speedier Long is already motoring around 2nd and heading for 3rd. Spencer's only chance is to try to get the lumbering Giambi on the shorter throw to home.
Spencer in RF overthrows the cutoff man, Martinez at 1B, and the throw is weak and offline. Mussina, who should be backing up Tino, is nowhere to be seen. Jeter has no reason to cover 2nd (Long is going full tilt toward 3rd) and has cruised into position to back up Spencer's throw, just in case. When the throw does prove to be rather weak and off-line—Jeter fielded it in foul territory—Jeter sprints for it, nabs it, and does a quick turn and accurate flip to Posada at the plate. Posada makes a quick turn and tag to nab Giambi.
Now, what I see when I study the film is that Jeter is showing outstanding game sense and anticipation. He's taken into account that Giambi is slow enough to be caught at the plate. He's noted the flight of the ball and realizes that Spencer has a tough throw from the deep RF corner. He knows Spencer has a dicey arm. He has no reason to cover second. He senses that Mussina is not where he should be, backing up the play. Jeter, having no other role, has moved into position to back up if needed. When he sees that the cutoff man has been missed and that Mussina is definitely not there, he sprints for the ball, whirls, and makes the quick flip (like a double play feed--so he's practiced it) to Posada.
It takes longer to describe the play than it did to make it, but what I see when I watch the play is that Jeter is deeply in the game, and ready to do what is necessary to win if the moment arises. That play definitely didn't happen by accident.