Local Ko Threat
A local ko threat – also known as an internal ko threat – is a ko threat which cannot be ignored because the follow-up would make winning the ko irrelevant either 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 a local 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.
Black is playing ko for the life of his corner group. He just captured the ko with . The point a now is a local ko threat for White. Black must answer at b and let White retake. If Black ignores her threat and connects, White will kill the black group with b.