Sensei is doing a blog post series on basic aikido principles. This one talks about unbendable arm. It was a really difficult concept for me to get at first - mostly because it's as much mental as it is physical. I started taking tai chi after doing aikido for 2.5 years, and knowing about unbendable arm from aikido really helped my tai chi practice also.
https://spiritualgravity.wordpress.c...nbendable-arm/