To perform it, a fighter has to stiffen his body to it's utmost limits on the moment of impact. When fighter uses it at the right moment during a seiken punch, all of his joints will be fixed. It's very difficult to do, but if he succed, he's fist will strike with the weight of his whole body.
At the first time, Baki used the Goutaijutsu against Kureha Shinogi during their fight at the Tokyo Dome underground arena. He's almost killed Kureha with this technique.
He used it also when he fights Biscuit Oliva during the Prison Battle Saga.