The "shadow" side of the double expresses Gene's mixed feelings about Finny from the start. Some critics have identified Finny as Gene's "Doppelganger," another self, wild and uncontrollable, that Gene loves but feels he must destroy. Gene is the good boy, the theory explains, the student who wants to obey, but is prevented by dark forces beyond his control, represented by Finny.
Throughout the novel, Gene's preference for an orderly life is disrupted by Finny's whims, impulsive and dangerous. As much as Gene enjoys these occasional thrills, he feels threatened — both academically and personally — by Finny's freedom. At one point, Gene even becomes convinced that Finny's outings and forbidden jaunts are a deliberate attempt to sabotage Gene's plans to become the valedictorian. Since Gene's academic ambitions are so close to his heart, so crucial a part of his self-image, the suspicion horrifies and angers him.
Given this tension, Gene's instinctive jouncing of the limb might represent a kind of self-defense: an unconscious attempt to destroy, or at least to cripple, a dangerous, uncontrollable part of himself — his shadow self. Gene's action, then, takes away Finny's power to disrupt Gene's orderly progress towards conventional adulthood. After the fall, Gene does not have to fear the consequences of Finny's unthinking action. The irony, of course, is that Gene's own unthinking action will have terrible consequences of its own.
