What is the significance of the grandfather's dying speech in relation to the narrator's speech/boxing match?

In Ralph Ellison’s Invisible
Man, the narrator is troubled by the words of his dying
grandfather. In essence, his grandfather advises him to perpetuate dual existence – an
outward conventionality and an inward rebellion. He instructs him to always offer
courteous and submissive responses to whites, while inwardly scheming to overcome the
oppression that they impose on blacks.  Initially, the narrator dismisses his
grandfather’s words as idiotic rambling.

On the
night of the battle royal, the narrator is forced to at least contemplate his
grandfather’s words. When the young black men are blindfolded and thrown together to
viciously fight one another for the white men’s pleasure, the narrator is mortified.
Still, he composes himself and delivers his graduation speech to the boisterous crowd.
 In this instance, he offers words of submission and humility despite his feelings of
disappointment and dissatisfaction.  He behaves contentedly although he suffers in
misery. The audience, composed of white men, accepts the duplicity and rewards it with a
gift and a scholarship to college.