Probably the most common element of hypnosis an intense level of focus. When James Braid coined the term "hypnosis" in 1841 he quickly realized that wasn't quite the right name, since hypnosis has very little to do with "hypno", or sleep. He tried to rename the state "monoideaism" or "single focus", but it was too late, "hypnosis" had caught on. People who have been hypnotized report having been able to concentate with singular effectiveness, often to the exclusion of everything else.
In the hypnotic state, we call it "trance" as a sort of shorthand, much of the critical factor of the conscious mind has been bypassed. That doesn't mean that that the hypnotized will fall for anything, they won't, but rather that a principle we call "trance logic" is in play. You won't fork over your deepest secrets, but you might be willing to connect the dots leading you down the path to beleive that going to the gym is going to be a fun adventure that you have actually been looking forward to experiencing for a long time.
Hypnotized people have an easy time letting things go. It's easy to take a step back and watch a reply of something that got you really worked up, but do so as an observer and actually create a commentary on what might have been done differently. Imagine experiencing something very stressful. Now imagine watching that same thing on TV. That is the same sort of detachment that many people experience in hypnosis.
I LOVE this one. Synesthesia is common. Synesthesia is a mixing up of your senses. Some people can "see" sounds while in trance. Others experience smells as sounds. Synesthesia doesn't happen for everyone, but it's pretty common and even if you don't experience it, sounds are often more intens, colors brighter, light often has "halos" around it and so on.
Okay, hypnosis is not relaxation and relaxation is not hypnosis. Hypnosis can occur in ways that would seem to preclude relaxtion, like onstage in front of hundreds of people. But, in a therapy environment, relaxation is the number one sensation that most people report feeling. Cares and troubles fade into the distance and just don't matter anymore.