This happens on the launch. Before the ship even leaves the earth, Ender understands that without gravity, what they are using as a floor could very easily be a wall of the ship in space, or the ceiling. He begins to imagine (from his seat) that instead of walking down the aisle upright, others are hanging upside down. Then he imagines that in his seat, HE is upside down. Basically, this ability to mentally change his perspective, shows that he is able to think outside the box - even before the problem is fully presented.
You will find as you read on how he uses this in the battle room. Until he gets there, all of the armies maintain the same orientation in the battle room that they did in the corridor outside the room. Ender comes up with a new strategy that becomes key, not only for winning battles against armies at battle school, but beyond as well.