It's just a flow like water through a hosepipe.
It seems a very accurate discription for the unbendable arm. In fact it is a test. A test to see if your flow of Ki can hold its ground against an applied force. But it is a static test, it is not a way to do a technique, but a way to practice the Ki and Kokyu principles in order to use them during waza. So in that manner it is important to make sure that you don't resist using any muscular strength, only the flow of Ki.
On the other hand during waza,you have to be moving, so you will use your muscles in order to be able to move your arms, your feet and your hips. But the use of muscles must remain to that and not be used as a resisting force against the Uke, or in order to throw him or pin him.
I know that there are people, some of them were close to o'sensei, who claim that there is no Ki, that o'sensei's advantage was always due to muscular training, isometrics or whatever. But the fact of the matter is that o'sensei was always teaching the use of Ki, the balance between IN and YO and it is quite clear from recorded evidence that he was very vigorous and effective even at an age where to speak for muscular strength would be ridiculous to say the least. So i'm very sceptical towards these people no matter how important they are supposed to be.
In my opinion in aikido you have to develope simultaneously the physical, spiritual, and mental self.Isometrics, weight lifting, aerobic e.t.c are subjects for sports men and not martial artists.