No! Not the horse. The term that many fly tiers use to bolster their claim about a fly pattern or design that they have 'invented' (or whatever word you want to use) and causes the trout to hit that particular fly pattern. Now, do trout strike because: of 'triggers'; the 'impression' of the fly; the 'imitative' likeliness of the fly; the 'presentation'; color; size; shape; All of the above. None of the above. Some of the above?
Why is it that at the same location, on the same river, on two succeeding days with almost the identical weather pattern and river conditions, at the same time, same hatch, same flies, etc. you have great success and then get skunked?
I'd really like to hear if anyone really thinks that a particular 'something' works as a trigger. And if it does, why does it not work all the time. And lastly, if you're that sure about 'that 'trigger', then there's only 1 reason to have flies that are another design - you don't really have that much faith in that 'trigger' now do you?

Sorry, but this is a subject I'm always interested in and have found it interesting to discuss, especially with fly fishers who strongly agree with the concept yet, have several hundred or more flies in their vest.
All the above written in good humor and opinions are just that.

Allan