...although there is a sort of parlor trick associated with it.
Photo courtesy of Josh Smith
Similar to breaking boards across a karate-guy's shins or bending iron bars against a kung-fu guy's throat, these are demonstrations of phenomena that might be of some use in some real context, but as typically demonstrated, amount to not much more than a parlor trick to wow the crowd. Ueshiba had several of these, like the unmovable body or the unliftable body or the unbendable arm.
Chris Marshall wrote an interesting article a while back sort of 'de-bunking' the unbendable arm parlor trick. You ought to read it because it is very entertaining - especially the video - but keep in mind the real unbendable arm is not just a parlor trick. It is also an integral part of the way that aikidoka are taught to generate and apply force and to slip out of the way of the opponents' force.
The big secret is that paying too much attention to making the arm unbendable is getting the cart before the horse. That is, getting the concept backwards. The big secret is...
If you move properly, you don't ever have to bend the arm. The unbendable arm in this context of proper whole-body movement becomes either an automatic separator or an instantaneous force-transmitter.
Patrick Parker, is a Christian, husband, father, judo and aikido teacher, Program Director for a Cardiac Rehab, and a Ph.D. Contact: firstname.lastname@example.org or phone 601.248.7282