The quote above is a clue into Ender's character, more than anything. The fact that he is sitting on the sidelines for a moment, just observing the others is clue number one. It foreshadows one of Ender's strongest characteristics as a leader. He is able to watch others, see their strengths and weaknesses, and use them to his own advantage as well as the advantage of the team.
When he notices how Bernard tightens up in flight, he is also making a mental note of a battle room tactic, which is to remain relaxed and fluid. Ender will use this bit of information not only physically, as he learns to control his body in the battle room, but he'll also apply it strategically, when as a commander he'll guide his army with less rigidity than other commanders.
It is the beginning of this paragraph, however, which holds the most important clue to future events in the novel, especially where Ender's leadership is concerned. The paragraph starts with:
Ender watched them cross the huge room, Bernard struggling to orient himself to the direction he thought of as the floor, Alai surrendering to the movement...
The information Ender stores away here is the idea that Bernard struggles to orient himself to the "direction he thought of as the floor." This is one of the first things Ender completely changes about the mentality of the battle room. Up until his arrival, all armies and soldiers emerge from their gate and use the gravitational orientation of the corridor they emerge from as their sense of up and down. Ender's new tactic is to train his army to always orient themselves in a position where the enemy's gate is down. Not only does this give him immense success in the battle room, but later, it is the same orientation which allows him to attack the enemy's planet by descending upon it.