I should revise my statement: "Hidden Techniques" is probably not an imprecise translation, but I feel it can be imprecisely interpreted.
The idea of 奥義 and 極意 is that they are not on the surface. As Soon-Kian as suggested, they are at the "heart". They are like the center of an onion. They are "hidden" in the sense of "not easily seen", not exactly in the sense of "kept secret, hidden by somebody".
This can often be idiomatically rendered in English as secret, e.g., "the secret of my success". In this case we are not really talking about a secret kept from other people, but rather a non-obvious, perhaps non-observable cause, key, or way of doing things.
Ueshiba, for example, once said: "合気道の極意は、己を宇宙の動きと調和させ" "Aikido no gokui wa, onore wo uchuu no ugoki to chouwa sase", "The secret (gokui) of aikido is to match one's self with the movement of the universe..."
It was this nuance of 奥義 and 極意 that I wanted to convey. I apologize for not explaining myself clearly earlier.