When things are going badly wrong, the hero needs to hit something to prove he's a badass, but can't hit anyone because that would be unheroic. So he hits something inanimate. His pain proves he is sincere. Done by a guy as a sign of Testosterone Poisoning, but also common among Tsundere and people who are simply approaching the limit of what they can take. It can also be the first sign that you're staring into the face of Mama Bear. Tends to avert Invulnerable Knuckles, but not always. It's also a common reaction to Rage Against the Reflection towards mirrors affixed to walls.