Think of that run the Rockies made in '07 to make the playoffs and cruise to the WS, where they met the Sox and were stopped hard.
Nobody is claiming momentum lasts forever just as slumping teams rarely stay slumping for super long times. If they suck, sure, but then it's not really a slump, is it?
The Sox have gone on a couple incredible stretches this year. You could feel they knew they were going to find a way to win. They've also had some lulls where they look like they are grasping at straws.
As a player, I have felt both feelings, and I truly believe that which mindset you have is a big part of the game. Of course, there are 25 players on a baseball team, and not everyone is in the same mindset at the same time, but winning 7 out of 8 most likely puts most in the positive mindset.