His exact words were (in English) "this is the secret of Aikido".
And now this is really where I get off.
He said the secret of Aikido was to do with the natural universal movements and he also said the secret of Aikido was not how to move your feet but how to move your mind for he teaches nonviolence. He said the secret of was thus many things. He said the secret when taking on tenryu the sumo was the circle.
So it's best when speaking English not to use the term secret at all unless you're trying to sell newspapers because it sounds just like a headline attention grabbing statement. Best use the proper term rather than the mystical intriguing term and that would be principle.
Then put that in context by saying it is a principle rather than the principle.