Hugh and Gerardo,
You are engaging in Textbook circular reasoning. You believe that everything O Sensei did was based on this notion of in yo ho. Therefore you see Sensei throwing with the fan in one hand and the other dangling as evidence of in yo ho because your approach also calls for the hand to be down to balance the forces in the body. You ignore all the examples when his arms are not as you desire. I understand what is being claimed. I call it dangling to make a point. Beginners always leave an arm dead. I guess you'd say they know the secret of in yo ho.
Much of the time O Sensei threw without touching. The principles on display on those occasions were entering, timing, blending, and taking the mind.
That O Sensei did a variety of things to fit the situation is not an opinion. You can see this on the videos. He described what he was doing to his students. He gave interviews. He wrote. It's called take Musu aiki.
The arm is not dangling but it's connected and extending energy. The one arm up and one down is a position - a pose - to demonstrate understanding of certain principles (in a sense O Sensei was
showing off... ), and is a known pose that has a historical significance. However you can maintain the same principles with the arms in different positions. In yo ho is not only shown through this pose but through every movement of the body; there are other tell tale signs of such training and conditioning.
O Sensei did many things, some of them like bayonet, spear training, misogi, etc. that almost nobody does today. And yes, sometimes he is shown doing things like no-touch throws, blending and apparently leading his uke's mind. The efforts to put this and everything else in perspective and better understand what he was doing, what did and did not teach, and why, are still ongoing. In the process I've come to share the view that this sort of practice of blending, leading, and large taisabaki (at least as codified by the second Doshu and other deshi) does not fullfill aikido's promise of resolving conflict through non-violence. It's fairly evident that most aikidoka cannot deal with skilled attacks or high-pressure scenarios using aiki-do with only these tools. I am not saying that this training is wrong or that it has no value, I'm just saying that it's insufficient to train O Sensei's aiki and therefore do his aikido. In fact replicating this type of movement from the outset can hide a fundamental deficiency: You can have beautiful, seemingly harmonious external movements, perfect timing, perfect technique... and yet at the same time you can display collisions, inbalances in the body, and force-vs-force events; in other words the antithesis of aiki. You can see this in 99% of the aikido videos out there.