Re: Aikido Principles: Unbendable Arm
Ok, it is a party trick. And Demetrio's comment is valid too - my understanding is Tohei developed this as a quickly learned example of how the mind can affect the body for more power in movement. This exercise was created as part of a shortcut, compared to O Sensei's teaching style. The imagery helps take antagonistic muscles out of the posture and helps with some alignment issues.
I do take it a bit further and lean in the outstretched hand from in front, and make the point that proper alignment goes down to the feet.
Having "got it" once in a stress free environment, eventually this becomes something like shomenate in motion and with proper timing and that takes practice to become a default alignment.Shodokan and Yoshinkan seem to do very similar things in a slightly different way.