Throughout history, go (wei chi, baduk, etc.) has been considered by some to be a martial art in its own right. Therefore, it is not surprising that the term kiai ( 気合い ) is often used in commentary related to go games. Twentieth Century go master Go Seigen 9 dan used it often in his commentaries as have many others. Used in this context, kiai often translates as "fighting spirit" or aggressiveness or initiative. Kiai means keeping sente, not letting the opponent have his or her way.
I have heard the term used in the following ways. First, I have heard the term used in coaching. "You need to put some kiai in your invasions. You are playing much too passively." I have also told myself kiai as a mantra during games.
Examples of kiai moves would be things like answering a kikashi -- forcing move -- with an unexpected move, snatching sente away from the opponent or breaking through and counter-attacking. Kiai moves are bold and irresistable. They turn the tables and catch your opponent off balance. They are the opposite of passive or submissive. Rather than following your opponent around the board and answering her every move, rather than being reactive, you dictate the direction of play.
Think Patton. Think Bruce Lee. They had the kiai attitude.