The unbendable arm isn't a trick as long as you take steps to make sure people don't turn the arm for advantage. We have gone over this before in class as part of a discussion of Sanchin. The arm isn't really unbendable as I'm sure most here know but when you have the right focus and concentration you can very significantly increase the amount of force it take by somebody to bend your arm.
This was used to help explain how our balance and focus should be for the part of Sanchin where you do the slow palm push outs. (atleast we do it that way) Your suppose to be focused and be able to root into the stance and be able to push somebody back from you. Very very hard to do without intense concentration if they resist even a little. If you focus properly you can push pretty darn hard.