I’ve written a few times that the best way to pace a 400 IM is to swim negative splits on the last three legs — i.e. to swim the 2nd 50 faster than the 1st 50 in Back, Breast and Free. This accomplishes two things:
1) Allows the neuromuscular system to "reset" for the new movement pattern. In the 400 IM, you alternate between short axis rotation in the odd 100s and long axis rotation in the even 100s. It’s a completely different movement pattern and you can make the shift most efficiently if you don’t press the pace immediately as you start a new stroke.
2) It creates an encouraging feeling of building momentum within each leg.

Though I haven’t seen the splits for the men’s Olympic 400 IM final published yet, it looked as if negative splitting within each 100 was a key part of Phelps’s strategy. I’ll be interested to see a comparison of Phelps’s splits vs those of Ryan Lochte and Laszlo Cseh.

Prior to the race, many expected that Lochte’s best chance at winning lay in his superiority to Phelps in the breaststroke leg. Phelps was expected to lead after the fly, losing his lead to Lochte on the back (Lochte is world champion in the 200 Back, though Phelps is nearly as fast). To win the race, Lochte needed to open enough of a gap during breaststroke to hold off Phelps’s powerful finish in freestyle.

Instead, after Phelps led the fly, Lochte moved in front on the 1st 50 of back, but Phelps seemed to regain a slight edge on the 2nd 50. Lochte swam about even with Phelps on the 1st 50 of breast, but Phelps opened a significant advantage on the 2nd 50, which put the race out of reach, because no other IM’er can match his freestyle at the end.