Now I can't find it explicitly, which raises the question whether I made it up/analyzed what was there and conflated it with canon... I mean, we do know that they could permanently alter the Gods World, and Sorcery was their preferred tool (and what the Guide's definition makes it sound like).
Anyway, the Guide still has one explicit definition: "The secret is dead with its initiates, but was evidently called the RuneQuest Sight. It apparently allowed initiates to see the world as a series of patterns, relationships, and repetitive reflections which could be organized according to the now famous Runes. Their Heroquesters followed the paths of their Runes through the Otherworld, and then shaped the Otherworld by planting those Runes into other parts of it."
Combine it with the other definition in the Guide: "A man whose name is lost made a discovery thought impossible. Mixing magic was always a dream of the God Learners, but seemed to be impossible due to some mysterious internal structure of the world. Yet, around 700, that forgotten person did it: he learned how to use sorcerous manipulation to alter divine Rune magic."
Which are both in complete harmony with the text you quoted. Taken together, it seems reasonably clear to me that the Secret was all about applying a naturalistic sorcerous world-view and methodology to both the physical world and the God World.
That said, I'm super unclear on the relationship between the God World and the Hero Plane(s). They keep sounding like very different things, but I'm not sure how they differ so fundamentally. HW made it seem as though when you go to Orlanth's Hall, you're in the God World, and then you can exit from there to Hero Planes.