Local Ko Threat
A local ko threat is a ko threat which cannot be ignored, because the follow-up would make winning the ko irrelevant by killing (Example 1) or saving (Example 2) the stones, over which the ko is being fought. This kind of threat should therefore be played before any other kind, the exception being when it is a loss-making threat. If your opponent has such a threat, it will be to your advantage to remove it, if possible, before the ko fight begins; see an example of removing a local threat.
Local ko threats are sometimes also called internal ko threats.
In this diagram, Black is playing ko for the life of his group in the corner. Black has just captured the ko with . The point a is now a local ko threat for White. If Black ignores this and connects the ko, White will play b, killing the black group. So Black must play at b himself to avoid this. White can now retake the ko.
In this example, Black is fighting a ko for the life of the group. When takes the ko, we can see that Black has a local ko threat at a (atari two White stones).
The Japanese for local ko threat, soba ko, literally means "ko (threat) nearby/at the side".
More examples can be found in Ko fight example from a pro game - 3