I feel like this is one of those instances where, once everyone knows about it, everyone does it, and everyone knows why you're doing it. Therefore, it eventually becomes ineffective (perhaps there is a name for this phenomenon). If I keep seeing books/blog posts/news segments on "power poses" then every interviewee starts putting their feet on my table, surely I'll notice. It's the body language version of a buzzword.
Also Zed Shaw had an interesting take on putting your feet on the table, but the link appears to be dead now. See: https://news.ycombinator.com/item?id=1235791