Maniac won his race with Mars Bar running backwards, which was insulting to his opponent.
Maniac just did not understand what he was getting into when he met Mars Bar. He didn’t realize that the boy had a reputation for being tough and that Mars Bar was a threat to him. His first mistake was not acknowledging Mars Bar's greatness, and his second was taking a bite out of his candy bar.
Maniac was really confused by Mars Bar. Just as race was not a concept he understood, toughness was lost on him. He made an enemy of Mars Bar though, and he realized that. After Grayson's death, Maniac returned and ran into Mars Bar again.
Mars Bar told him he had new sneakers, he had been working out, and he was a faster runner. He challenged him to a race. Maniac was not sure what to do.
Even as the race began -- even after it began -- Maniac wasn't sure how to run it. Naturally he wanted to win, or at least to do his best. All his instincts told him that. But there were other considerations: whom he was racing against, and where, and what the consequences might be if he won. (Ch. 38)
Maniac’s decision to run backwards was an unusual solution to a difficult problem. He wasn’t really trying to humiliate Mars Bar by proving that he could run faster backwards than his opponent could run forwards. He just didn’t know what to do and it was the best he could come up with.
Why did I do it? was all Maniac could think. He hadn't even realized it till he crossed the line, and he regretted it instantly. Wasn't it enough just to win? Did he have to disgrace his opponent as well? Had he done it deliberately, to pay back Mars Bar for all his nastiness! (Ch. 38)
In reality, Maniac just loved to run. He enjoyed being on the East End among the people there. He really missed it and the Beales. He wished he had a home. Maniac had again added to his legend without meaning to.