What is the "blind impulse" Finny says the narrator had?

Finny bursts into tears and says that some sort of blind impulse must have seized Gene on the tree those many months ago, that he hadn’t known what he was doing. He asks Gene to confirm that it was some impulse, not some deep feeling against Finny, that took hold of him that day; Gene answers that some “ignorance” or “crazy thing” inside him made him jostle the limb. Finny assures him that he understands and believes Gene.