An example of removing a local threat

White should play a preliminary move  

White can only win this fight via ko: she has too few liberties to win a capturing race.

However, before taking the ko, White should play , forcing Black to answer at a. Otherwise, when she starts the ko, he can play at himself as his first threat. (There is no reason for her to play at a, reducing the size of the ko.)

Since this local ko threat is also a mutual ko threat, this illustrates the principle of removing mutual threats before starting a ko. Had it only been a threat for White, she would have done better to play it as her first threat in the actual fight. Had it only been a threat for Black, he would not have had to answer an attempt to remove it, so White would have had to start the ko and he would have been able to use it as his first threat.