I know very little on the rules of making the elbow bend and I believe Ki does exist. However, (I have nothing but respect for these people) I just wanted to say it is fairly simple to lock your elbow. If you want to bend it just strike the inside of the elbow while pulling the arm. (You dont have to strike too hard)
Sorry but that doesn't work if you are doing unbendable arm correctly. As I said, it's a misunderstood exercise, at no time should your elbow ever be 'locked'. Striking it doesn't help in the slightest (except perhaps for beginners).