In order to use Zettai Maken, the user must be able to control all the chi in their body and focus it into one point. The user must think of the strongest person or thing they know and the technique will be a picture of that. It causes a fearful effect that scares enemies and forces them to surrender.
The user must focus a lot and believe that they will definitely win, as the least amount doubt ruins the technique and fails. Thus, a successful Zettai Maken guarantees victory and, as Reborn said, winning without even fighting is a great strategy.
When Reborn attempted 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 Takeshi Yamamoto, Ryohei Sasagawa and Hayato Gokudera.
Reborn made a mammoth and roared and all three of them couldn't do anything about it. After, the mammoth disappeared. 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 how to control their spirit.