The fun part is watching people actually try to control their body language in reality. You can try, but if you aren't feeling what that "power pose" is broadcasting, you'll just look like an idiot to anyone even remotely paying attention. You aren't going to gain leadership by leaning back and spread eagling in a business meeting. You're going to gain it by being an actual leader and doing stuff that adds value to a…
I sort of agree. I find that avoiding bad sorts of physical signals is definitely useful, but consciously instantiating new signals in a convincing way is (surprisingly) very difficult.
That said, there are a few people who are really good at this stuff. One technique is based more on directly altering your mood rather than consciously positioning your body, and some people just seem better able to manipulate their mood than others.