Why blocks comprise only one movement

The issue Last time I promised to explain exactly why "blocks" comprise just one movement and not two. But before I do that, I must first set out what is meant by "one movement" and "two movements" in this context. First, I want to make it clear that I'm not talking about two different arms moving (which I covered in my article " Two for the price of one: more about karate blocks "). Rather, I'm talking about two movements off one arm , intended to deflect or otherwise stop an incoming attack. In a previous article I related this anecdote: I recall as a young martial arts teacher being confronted with this issue when a rank beginner asked me: "What stops me from just hitting you as your arm goes to the side?" The beginner in this instance was talking about the chudan uke or "chest level block" but he might as well have been talking about many of the others. I recall he smirked as he demons...