How does Brinker stage a "mock arrest" of gene?

In Chapter 7 of A Separate Peace, Brinker stages a "mock
arrest" of Gene as they go to the Butt Room, a basement area where the boys smoke. 
Seizing Gene's neck, and pusing him into the Butt Room, Brinker
"announces,"

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Here's your prisoner, gentlemen...I 'm turning
him over to the proper
authorities.

One of the boys
asks what the charge is, and Brinker replies,

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Doing away with his roommate so he could have a
whole room to himself.  Rankest treachery....Practically
fratiricide.

Gene shakes
himself free from Brinker, who tells him he will have his day in court.  Gene then
says,

Shut
up!  I swear you ride a joke longer than anybody I
know.

This statement is a
mistake, Gene narrates becaus the radio suddenly became quiet and his voice rings out,
"galvanizing them all."  One boy asks, "So you killed him, did
you?"

Brinker qualifies his
statement:

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Well, not actually killed.  Finny's hangin
between life and death at home, in the arms of his grief-stricken old
mother.

Gene tries to diffuse
the situation by saying he just a "little pinch of arsenic in his morning coffee." 
Brinker will not let Gene out of the situation by declaring "We know the scene of the
crime."  Again Gene tries for humor, using a "funny French pronunciation":  "Yes, huh,
yes there was a small, a little contretemps at the
tree."

When a young boy asks Gene to tell them everything,
there is an "unsettling current in his voice, as though he believed everything," so Gene
gains confidence and continues his fabrication, saying he stole Finny's money, cheated
on his entrance test, blackmailing his parents; then, he made love to his sister....,but
when he can easily slip in the truth and have it disbelieved, Gene's throat closes on
him:  "I could never say them, never.

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I swung on the younger boy. 'What did I do
then?' I demanded.  I'll bet yhou've got a lot of theories.  Come on, reconstruct the
crime.....

His eyes sung guiltily back and forth, 'Then,
you just pushed him off, I'll
bet'

After getting the boy to
say the truth that he cannot, Gene has effectively defeated the crowd.  However, as they
go up the stairs one boy remarks,

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Funny, he[Finny] came all the way down here and
didn't even have a
smoke.

What begins as a mock
arrest almost becomes a trial for Gene.  In fact, it has for
Phineas.