I do not believe relying on beginner's "unbendable arm" is at all helpful. Beginners cannot maintain unbendable arm reliably, leading to the too-damn-common newbie injury, shoulder separation at the acromioclavicular joint. Some of us are permanently in pain or impinged function. Others simply disappear.
I second the endorsement for Ellis Amdur's method and also the use of large balls or cylindrical rolling tools. If somebody is scared, the teacher can hold on to control the speed of the turning of the ball or cylinder.
In my Falling class, for non-martial artists, we do a very slow 6 session progression that never uses unbendable arm; it would be easy to teach it afterward. It is documented here:
https://www.youtube.com/playlist?lis...jVVAgi6a5zq9Jz