I hadn't really thought of it in just that way but, yeah, "emergency recall" is how I train dogs offleash as well. That special command that's given when they absolutely, positively MUST come back instantly. My usual recall is just the dog's name with the word "come". I use the same method used in military drill commands. The name is given as preperatory and the command comes louder and more forcefully. With the emergency recall I simply make the command louder, sterner and add the word "now" with an even louder and more forceful tone than the original command so that it comes out something like this:
Frigga, <span style='font-size: 14pt'>COME </span> <span style='font-size: 20pt'> NOW! </span>
Frigga, <span style='font-size: 14pt'>COME </span> <span style='font-size: 20pt'> NOW! </span>