An example of removing a local threat
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.
P.J.Traill: Could it not make sense to make the ko smaller if she expects to lose it, and just wants the profit from Black ignoring her threat?
Dave: Good question but be careful of what we mean by the "size" of the ko. Changing to "a" and forcing Black to play at 1 instead of "a" will take one Black stone out of the ko. This will reduce the cost of losing the ko for Black but will not reduce the cost of losing the ko for White. Therefore playing at 1 is correct.