Zettai Maken (絶対魔拳) is a technique Reborn tried to teach Tsuna, Kyoko and Haru when they had to deal with dojo-breakers. In order to use Zettai Maken you have to control all of the chi in your body and focus it onto one point. When one uses it, it causes a fearful effect that scares enemies and forces them to surrender. However, in order to succeed, the user must focus a lot and believe that they will definitely win. Even the least doubt, ruins the technique and fails. Thus, a successful Zettai Maken guarantees victory and, as Reborn said, wining without even fighting is a great strategy.
When Reborn was trying to teach Zettai Maken to the trio, they went through hard training that increased both their physical condition and their spirit. After they were done Reborn showed them what it was about and used it on Yamamoto, Ryohei and Gokudera. All three of them admitted defeat before they could do anything. When Tsuna first tried it, he failed because he wasn't 100% sure about himself. Kyoko and Haru never really used the technique, however the training taught them well how to control their spirit.
"Zettai Maken" can mean "absolute evil fist" or "won't lose."
According to Fuuta, it is ranked 3 in martial arts that should be sealed away in the 4000 year history of China.