I like the metaphor for safe word, you still drive carefully even with a safety belt its j...

Yeah, same. But the problem here is that most vanilla people do not grasp the crux of BDSM plays or understand the planning that goes into it. Most people think spanking, binding, rough sex and sometimes using candle wax is the extent of it. Plays that can cause severe physical injuries if you go a tad too far aren't in their realm of possibilities. Even for this couple, the recent one was their first experience with choking but neither has talked about it seriously. They need someonethat could tell them what could go wrong and how fast before they make a huge mistake.