Does anyone find it interesting how often teams perform well the first week or 2 for an interim coach? They almost always seem to get an immediate impact with a new head coach almost as if there's a weight lifted from the team. To me that speaks for how much of this game at this level is mental rather than physical. A good head coach lifts your team, whereas a poor head coach drags it down.
That could be some of it. The other part could be similar to the phenomena that often a new QB has a few really good games before regressing. New coach adds one or two new wrinkles to the game plan the opposing coach isn't expecting once his new ideas are on film they stop working.