An example of removing a local threat

White should play a preliminary move  

White can only win this fight via the ko: White is too short of liberties to make it a capturing race.

Before first capturing the ko White needs to play at , which will make Black answer at a. Then White can start the ko. There is no reason here for White to play at a, reducing the size of the ko.

This is a special case of the principle that you remove double threats before starting the ko.