Many years ago there was a Karate demonstration where the demonstrator did a straight punch to the partners face. The partner did a rising block to push the striking hand up out of the way and could not move the striking hand. This was a demonstration so the striking fist stopped just short of hitting the partner. It was quite amazing and we could not understand how it was done. Many years later I understood how to do this ''trick'' which is fundamental to Aikido technique. We don't usually talk about this so I am calling it immovable arm.
Watching the demonstration we thought that this was amazing. The attacker could have struck right through the block and have killed his partner if this had been full contact. Now I do not believe that would have been the case. This technique works by breaking your partners balance. You can easily try this by holding out your arm at shoulder level. This is similar to unbendable arm except that your hand is not supported. Now Uke tries to move nage's arm up. As Uke pushes up like a rising block nage slightly shifts back pulling Uke off balance. Like unbendable arm this is very subtle so that Uke does not realize what is really happening. Uke just experiences nage as being so strong that the arm cannot be raised. The truth is that because Uke is off balance Uke has very little strength. Once this is understood the amazing Karate demonstration is easy to perform. In Karate this was a trick because the hand pulls back. It would not have struck Uke. In Aikido this becomes the basis of much of what we do. We do what looks like a strike but when Uke reacts we pull back taking Uke off balance. We can then throw or strike Uke while Uke is off balance. We also routinely do this with Uke pushing in different directions. When Uke grabs our wrist and pushes down we do tenshim or just shift our hips back to prevent Uke from pushing the hand and to weaken Uke for our next move.