Can I suggest that the purpose of the unbendable arm is to use the structure to convey the forces on the elbow crease to the ground. IOW, the tester should feel like they're pushing against the ground - not the elbow crease.
It's not a trick, insofar as a demonstration of the body skill required in grounding certain forces at supposedly "weak" points in the anatomy.
I've read one particulr explanation of this trick as a demonstration of the antagonistic action of the triceps and biceps, but to me, it merely shows the lack of real understanding as to the purpose of this EXERCISE.
As far as I'm concerned, canting your arm so that the elbow crease is facing inward is NOT advisable, I've seen people almost pop the joint capsule as a result... not a nice way to have your arm broken....