A professional hypnotist once told me that it is very difficult to hypnotize "mathematicians" (him meaning: math, physics, and computer science students), because (this was his intepretation) they are too well connected with the reality and will not accept nonsense.

"Connected to reality" is in this context a nice way of saying that someone can't let go and relax. Computer Science/Physics/Math people especially have a problem with forgetting numbers because numbers are way more important for them then the usual person.

Also, he said that in order to learn hypnosis it is better to be hypnotized first, because then you know what it feels like to be hypnotized, and that knowledge is very useful when hypnotizing others (you have better intuition on what can and cannot work). -

That's not about having an intuition about what works. Part of hypnotising somebody else effectively involves going into a trance state yourself.