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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.