Chris has a lot of good articles on the Eight Powers, as well as some articles that touch on them. From my perspective, I think you'll get turned around if you look at Eight Powers and Six Directions together - they are not the same thing. Also, there is a fair amount of Chinese-based concept surrounding Eight Powers that suggests that it is probably not original to Ueshiba or even Japanese culture, but probably derived from China and re-interpreted. Read that as politely saying the Old Man got his ideas from somewhere.
Eight Powers is a sophisticated concept. I found myself confused about it for years, mostly because the only explanations I ever received where intellectual, not physical. In other words, I was told about it but nobody could ever really "show me." Starting small was a good place for me to start. For me, that was Three Powers - Heaven, Earth, Man:
Looking at another post, Chris ties together Heaven, Earth, Man and Eight Powers:
You get the idea that the numerology is not theory or philosophy, but a technical manual with instruction represented as numbers. Oh, it's 8 and
2, not 8 then
2, so Eight Powers challenges the whole "wave" thing that is prevalent in our movement. Instead, you're doing both simultaneously. Easy, right?