Re: to be imparted by oral instruction...
IMHO, the old styles hide everything in plain sight and the techniques must be stolen not explained.
You are probably right, that though not orally explained, it is in the way you are taught to move.
I could hazard a guess that arm power refers to the "extension of ki" (intent) into and connecting to your uke's center through ground force into your hips and through the "unbendable arm" while maintaining your structural alignment with your elbows extended, down, and in with your hands on your centerline while you move as one relax flowing body including your uke. Its a guess that was not obtained through oral instructions, so probably not what you are looking for.
Tell Mark and Sonya "hi" from an old kohai.