One of my favorite parts of hypnosis is the subject performing some unusual action after waking up and getting into cognitive dissonance about why they did it.

What causes those suggestions to stick with them and to have that effect?

Sometimes the hypnotist is able to further direct the subject by simply calling out the new suggestion and a finger snap without needing to return the subject to trance first.
How can you tell when that is likely to work?

I think of it kind of like a computer system. A personality is "built" from accumulating and integrating memories and experiences, each one building on the ones before, like making a photo album out of a stack of pictures. (*) Normally, this process only works one way, but putting someone into a hypnotic trance allows the hypnotist to access the stack directly and suppress things (the old 'forget about the number six' trick) or insert things, like a command trigger at the root of the stack. This is easiest if the hypnotist can get the trust of the subject, but it can also be accomplished with brute-force brainwashing methods.

(*) Obviously this isn't quite how the actual brain works. For one thing, our brains filter our senses, and will also go back and rewrite memories for the sake of preserving consistency, so it's not a stack of discrete experiences but more like a slightly-marbled, slightly-melty fudge layer cake...mmm, fudge!



I don't know about how to tell in general, but if I had a subject in a trance, and I wanted to maximize the odds of being successful in giving a new suggestion outside of trance, I'd want to take some time and lay the groundwork. Using that stack analogy from earlier, I'd add a new entry way deep in the stack to trust me and want to do whatever I said. If it's a stage show, play it up to the crowd by praising the subject for being such a wonderful participant. Or, if the subject came in skeptical before I entranced them, maybe plant the idea that, whether they want it or not, it's clear that they have no choice -- they won't be able to help themselves. Either way, make it a self-fulfilling prophecy.

Are you asking for the scientific answer? If so, you're probably better off asking an accredited psychologist or hypnotherapist. I'm not sure how many of them are among us, but looking at the real life jobs thread, it's very little.

If you're asking for the community head-canon about how fictional hypnosis works, then yeah, have to agree with mcfantastic, it works kind of like a computer.

We've had a few tists pass through, though, so it's not out of the possibility.

I am a layperson, but I reckon a good portion of it is that calling them "post-hypnotic" suggestions might be a bit of a misnomer.

Hi, hypnotist here. Not professionally, but have been practicing and studying hypnosis for well over a decade now.
Post-hypnotic *IS* appropriate naming, because it takes effect AFTER the subject wakes from hypnosis.

With that out of the way... to answer OP's question:
To understand hypnotic suggestion, either during or after hypnosis, it's best to understand what hypnosis is and isn't.
Hypnosis and trance aren't mutually exclusive. They often play off of one another, but one is not needed to achieve the other. That being said, they are often seen holding hands, and are often spending time together, so they may as well be kissing in a tree.
Anyways, being in a state of hypnosis, via trance or otherwise, sort of separates the conscious, subconscious, and unconscious parts of one's mind. This opens the subconscious mind to suggestion. The subconscious mind controls (in a sense) our perception of the world, and filters how we think. When given a hypnotic suggestion that it understands and can/will accept, it changes how the subject thinks to behave, act, or perceive the world around them (or themselves).
This effect can last past the point of hypnosis, hence the term post-hypnotic suggestion, but can fade over time; and in most cases, does fade over time, when not regularly reinforced or utilized. The mind makes way for more urgent/current matters, so unless it's reminded the suggestion is important, it WILL fade eventually. The duration of the effect is determined by hypnotist's suggestion, or the subject's mind, or both in tandem.
Often, strong but basic suggestions won't last beyond a good night's sleep.
Typically, when a subject is hypnotized, the hypnotist will say what time frame this effect will occur within. Be it while still hypnotized, or when the subject wakes up from hypnosis, or until it fades naturally, or whatever.

As for hypnotic triggers, those are treated as suggestions by the mind. Simply create an association with a word or action while the subject is hypnotized, or anchor it to a feeling/experience, and give a corresponding suggestion to go with it like you would give a suggestion normally, and voila, you have a trigger; more or less.
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Of course, everything I'm explaining is simplified and watered down a bit, but you probably get the idea.

I'm actually in the process of writing a sort of universal, simplified guide to hypnosis, applicable to both hypnotist and subject alike. So anyone interested can check that out when I eventually post it. I'll link it on the hub's forums when I eventually do share it. I may do it in chunks... We'll see.
Anyways, hope that helped clear things up a bit!

I will preface by saying I just have experience with such suggestions and trances, not a trained hypnotist.

I think the post by bullet above explains a lot of your likely questions, but to address specifically this, this is another form of post-hypnotic suggestion, just the suggestion is that when the subject hears the trigger (the finger snap) they will perform the new suggestion subconsciously.

I like to think of it like how when you’re playing a video game or driving a car, you don’t think about pressing the individual buttons on the controller or pressing the pedals, that’s your subconscious at work, taking a goal or idea and translating it into actual motions. Post-hypnotic suggestions have the power to give the subject instructions for motions without the conscious mind being aware.