Riosdad, take into account that this is demonstrated by, and for, english riders who ride with a contact. I have been doing something similar for years on horses who get strong, I figured out myself how to increase my strength, I didn't know it had a name :]
I also do it a bit differently - I don't necessarily lodge my hand on the neck. My emergency stop is just locking one hand in the most convenient position, and then hauling ass with the other. I don't need to shorten my reins. The uneven pressure means the horse can't lean back, I always do it in a snaffle, and I haven't had a runaway in years. If it doesn't work the first time, you do it again until you get the response. I don't 'teach' it or 'practise' it - It is simply a way to magnify the little strength I have by utitlising my body and the reins in the most effective way.