GNU Go Documentation
9.5.2 Threats to Attack or Defend
A threat to attack a worm, but where the worm can be defended is used as
a secondary move reason. This move reason can enhance the value of a
move so that it becomes sente. A threatening move without any other
justification can also be used as a ko threat. The same is true for a
move that threatens defense of a worm, but where the worm can still be
captured if the attacker doesn't tenuki.
Threats found by the owl code are called owl threats and they
have their own owl reasons.