According to the dictionary, here's the meaning of the word:
"A wire stretched close to the ground, working a trap, explosion, or alarm when disturbed and serving to detect or prevent people or animals entering an area."
So it seems like, when used for marketing, a tripwire would be something that alerts you the marketer to the presence of a possible customer. (It doesn't make sense that you would be wanting to prevent someone from becoming a customer.)
However, it seems to be used to denote an opt-in that starts the customer going through a funnel. That's what I don't understand. How does that relate to any of the meaning in the word "tripwire"? And why would we need a new word for that process?
What am I missing?
(a 35-year marketing veteran)