As an exercise I've noticed that because the bicep is relaxed its not working with the uke's effort to bend the arm, making the arm much harder to bend.
But here's the odd thing, I've also noticed that on really cold days when I'm out on a run - the same mental effort for unbendable arm make the blood flow into my hands so they don't feel cold any more. I think there's a bit more going on than just relaxed muscles.
They don't talk much about ki in my dojo either. But they do talk about relaxation, extension, and visulisation.