In a thesis presentation/defence you are talking to a jury who has already read your thesis.
In a conference talk, your goal is to get an audience who has mostly not read your paper interested enough to get the general ideas and read the paper in more detail if they care.
The thesis defence protocol depends on where you are, but if you get 20 minutes or so to present, then you don't have time to get into many technical details. However, since the jury has read the thesis, the presentation is mostly a formality, and is a chance for you to show them the big picture, and how your work instantiates the expected research methodology: problem - state of the art - solution - validation.
To "show off" you should have one or two slides explicitly listing your research contributions, and the publications derived from them (if any).
Regarding the problem of looking like you don't understand the math, don't worry about it. If the examiners are in any doubt about that, they will ask specific questions after you have finished presenting, and they will refer to the thesis, not to your presentation. if you have some ideas of what they will ask, then prepare extra slides to be able to answer those questions.
Here is also some great advice from a prof in my department. (note that the first 8 slides or so refer to the specifics of the protocol in use here, duration, jury composition, etc. But what comes after is more generic and useful for anyone, I believe. Also, take it as "do as I say, don't do as I do", since these slides are choke-full of text!)