In Chapter 7, Brinker stages a "mock arrest" of Gene when the two boys enter the "Butt Room," the basement room in the dormitory where students gather to smoke. As they approach the door, Brinker seizes Gene's neck and pushes him into the Butt Room ahead of him, announcing,
"Here's your prisoner, gentlemen...I'm turning him over to the proper authorities."
Brinker had accosted Gene in his dorm room just previously, and had brought up the subject of Finny's accident. In the course of their exchange, Brinker had jokingly accused Gene of purposefully causing Finny to get hurt in order to have the room to himself. The banter had become strained, since Gene had himself been struggling with the question of whether his role in the accident had been intentional or not, and to change the subject, Gene gets up and states his intention to go to the Butt Room. Brinker, however, will not let the topic drop, and carries it forward, facetiously responding, "Yes, yes. To the dungeon with you." Once the two boys get there, Brinker enacts the "mock arrest," and the students lounging in the Butt Room then continue the interrogation.
Brinker's "war poems,"
Is a bore,"
Is the core
of the war,"
reveal his apathy and detachment towards the war. The war has not yet had a significant impact on his life other than that he has had to participate in an apple-picking excursion because the regular pickers are otherwise occupied with the war effort. Brinker has not yet had to face the reality of war directly, and his slightly cynical poetry reflects this fact, that the war has not, to this point, made any notable demands on him (Chapter 7).
Brinker gathers everyone in the main hall in order to be present for the investigation and "trial," and proceeds to ask Gene and Finny a lot of questions about what happened at the tree that day. To make it seem even more like a trial, he calls in a first-hand witness to the events: Leper. Leper had been hanging around the school grounds of late, ever since his discharge, so Brinker probably approached him and told him to meet him in the building. Leper then "testifies" of what exactly happened that day, according to what he saw.
Leper reveals, in very poetic language, that Gene jounced the limb first, causing Finny to lose his balance and fall. This revelation, even though Gene has tried telling Finny this in the past, upsets the usually calm Finny so much that he curses and flees the room. It is his hasty retreat that leads to his next fall down the stairs, and his eventual death. It is an unfortunate thing, and Brinker was the one to gather everyone and drill Leper in regards to the original subject. I hope that those thoughts helped; good luck!