Learning how to do math, especially mental math, can be great, but it does come with a price. It frequently changes how perceive you.
You might think that math teachers, who want you to be good at math, would be exempt from this. However, many math teachers know the area of math they're teaching and just a few other areas. To paraphrase Will Rogers, if you get them off the topic in which they were educated, they're not as knowledgeable.
In his book, Secrets of Mental Math, Arthur Benjamin tells about how he discovered how to square two-digit numbers on his own while he was still young. The method itself was previously known, but he had discovered it independently. One day, in an algebra class, his teacher was working through a problem, and finished by writing the answer as 1082. Young Arthur Benjamin then blurted out that the answer was 11,664!
When he explained the method he was using, the teacher said she had never heard of that before, and young Arthur's mind quickly raced with the idea that he'd made a new discovery! He finishes by mentioning that, when he ran across the same method in a Martin Gardner book, it ruined his whole day.
With my interest in mental math and magic, I had a few similar experiences in my high school days. The first time you watch it, don't try to understand what he is explaining. Instead, imagine you're this guy's math teacher, and being boggled by what he's describing. It helped me finally understand what my math teachers must've gone through when dealing with me.
The worst part about the first time I watched this video was realizing that I could follow the techniques (largely because I'd used them myself before), and realizing how I probably sound when describing ideas like this to others.
If you want to understand the shortcuts he describes, go through the video bit-by-bit until you understand. You can find further help with my technique videos (most notably Arthur Benjamin's own Mathemagics). Sites like Curious Math, Mathpath, and BEATCALC can also help.
On a related note, TV's best known math geek, and his friends, are returning from hiatus after the writer's strike. The ad CBS used to announce the return of Numb3rs appeals to me on several levels. First, it demonstrates the perception non-mathematicians often have of people who are good at math. Second, as a Mac user, I couldn't help but appreciate that they're parodying Apple's well-known Mac vs. PC ads to grab your attention.