A trigger in ‘mind’ happens when there is an association between the casual event e.g. ‘a word’ being spoken, and the historic connections you have with that word. 

If you are seeking to achieve subtlety, then you cannot be too obvious, yet you still need the reference to hit the mark. ‘Obviousness’ is often not funny as there is no gap for the mind to leap. Take the example of the word ‘elementary’, for instance – I would bet 99.9%* of people would almost instantly get an association with Sherlock Holmes. 

*depending, of course.

Seeing the obviousness allows us to see what is not obvious, and seek out the subtle.