Seki collapse

Example  

The four black stones in the corner appear to be alive in seki. If the stones later die, the apparent seki collapses -- five white stones become alive, automatically killing the black corner. This change is called a seki collapse.

A seki with very asymmetrical stonecount can sometimes be used as losing ko threat by the player with the fewer stones in the seki. Since such a threat entails a loss, this makes sense only if the ko is really big. The opponent then may respond and capture the attacker`s part of the seki or take the ko and be captured. Either way, the seki is subject to a forced collapse.