空手无先手 先正其心

In the new trailer we can see Daniel explaining Robby that Karate is for defence only (空手无先手) and first learn rule number one (先正其心). Although this is technically correct many Chinese speakers here will under that 先正其心 as stated by Zengzi in "Great Learning" means a lot more than that.

In "Great Learning" Zengzi said "欲齐其家者先修其身。欲修其身者先正其心" or in English "If you want to take care of your family and your own, you must first shape your moral character and take care of yourself." In the context of his book Zengzi explained that "Those in the past who promoted virtue across all that's under heaven first shaped themselves to be a virtuous and balanced person first". So in essence 先正其心 does not just mean "karate is for defence only" but rather that "Karate is used for defence only, so to practice it (and teach it) you must shape yourself into a virtuous and balanced person first".

Although it is clear Daniel talks a great deal of balance it is very clear through the behaviour of his children and Robby that he didn't teach the lesson well. His daughter Sammy literally ditched her good friend for fame and refused to help her out despite suffering humiliation time and time again. His son is the stereotypical spoilt brat that needs discipline. And worse of all Robby still practices Karate for the unvirtuous purpose of pissing his dad off, and instead of telling him that "to practice karate one must first have pure intentions and not be a man who seeks to harm others" he just said "oh just make sure you use karate for self defence and everything else is okay".

Essentially Daniel taught his children and Robby that balance only applies to how to be deadly in Karate when the lesson he should have been teaching is that to succeed in anything in life, especially in karate you must be a virtuous and balanced person first. And to be such a person you must not seek to harm others for your own gain (which he, his daughter and Robby is guilty of) as the lack of virtuous intentions is what will ultimately lead you off course (like for instance buying a mall in blind rage just to get back at Johnny). Despite believing it himself, Daniel is far from a person who is 先正其心, and the lack of balance between self interest and virtue is why ultimately he will just be a hero in his own story, as opposed to the story of Cobra Kai.