The idea is simple. When people get into BDSM relationships of dominance and submission, sometimes the submissive party is so "in the moment" that they lose their ability to say much of anything. This altered state of consciousness can make safe words less useful. On the other hand, if we externalize and articulate a vision of the subconscious mind's fetishes, that can solve the problem.
A variant of the simple "traffic light" safety idea - you visualize all the fetishes in your mind as boxes. One might be green, meaning it can be easily and freely opened by your dominant and used. One might be yellow, meaning you're unsure, either you haven't done it or haven't done it enough or just want care to be exercised - this box is chained up, and requires one "key" - either you asking for it or them trying it. Then when you're trying it the box may become green or red depending on your opinion.
Red boxes need two keys, both of you agreeing, and if what's being done is hypnotic and some particular box comes up "red", you will stop right there anyway.
Furthermore since these visions are happening in your subconscious it might be easier to simply say "anal sex is a yellow box", "nipple play is green" and so on. Maybe the words will leak out as you moan, using "green" to mean "keep going please!" and so on. Good for people working with dominant partners - and you won't be worried about denying your partner since you're simply telling the truth, describing how you feel about a given play or idea. Whether aloud or written, you'll convey it, maybe just typing "oral service green" without thinking, or just "Green, green" moaning out of you as you cum, an enthusiastic encouragement.
In fact, providing knowledge of your favorite fetishes is like handing chains to your partner which can be used to bind, restrain, and train you - and that's just what you want, isn't it?
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