The reason a team plays great one day and poorly the next can be explained in two words - mental approach. The team's physical skills didn't change, but their mental approach did. Up until about a decade ago, a sports psychologist was someone athletes went to see only when they had a problem, not someone healthy and productive athletes and teams spent time with. No matter how good you are, you can always improve. The mental game is an important part of getting better.
Athletes spend hours physically practicing to get an edge on the competition.
Yet, what a team can really do to get an edge is right in front of their noses, or more accurately, right above their shoulders. The mind body connection is a very powerful one. For everything you think in your mind, your body has a reaction, regardless of whether it's real or imagined. Usually, you will wake up with your heart racing and body sweating and agitated, even though all you were doing was sleeping. In your mind, though, there was something bad going on and your body was reacting to it.