Needs more context. You and I know what this quote refers to; others might not.

EDIT: Here's a non-Tweeted version of the quote. It is used again later in the book, but to quote that scene would be a spoiler.

They finally got themselves together along the wall. Ender noticed that without exception they had lined up with their heads still in the direction they had been up in the corridor. So Ender deliberately took hold of what they were treating as a floor and dangled from it upside down. "Why are you upside down, soldiers?" he demanded.
Some of them started to turn the other way.
"Attention!" They held still. "I said why are you upside down!"
No one answered. They didn't know what he expected.
"I said why does every one of you have his feet in the air and his head toward the ground!"
Finally one of them spoke. "Sir, this is the direction we were in coming out of the door."
"Well what difference is that supposed to make! What difference does it make what the gravity was back in the corridor! Are we going to fight in the corridor? Is there any gravity here?"
No sir. No sir.
"From now on, you forget about gravity before you go through that door. The old gravity is gone, erased. Understand me? Whatever your gravity is when you get to the door, remember-the enemy's gate is down. Your feet are toward the enemy's gate. Up is toward your own gate. North is that way, south is that way, east is that way, west is-what way?"
They pointed.
"That's what I expected. The only process you've mastered is the process of elimination, and the only reason you've mastered that is because you can do it in the toilet. What was the circus I saw out here! Did you call that forming up? Did you call that flying? Now everybody, launch and form up on the ceiling! Right now! Move!"
As Ender expected, a good number of them instinctively launched, not toward the wall with the door in it, but toward the wall that Ender had called north, the direction that had been up when they were in the corridor. Of course they quickly realized their mistake, but too late-they had to wait to change things until they had rebounded off the north wall.
In the meantime, Ender was mentally grouping them into slow learners and fast learners. The littlest kid, the one who had been last out of the door, was the first to arrive at the correct wall, and he caught himself adroitly. They had been right to advance him. He'd do well.