The emergency 1 rein stop....bad or good?

It's important for both horse and rider safety. It's also important that a horse doesn't get away with bolting as that can turn into a very dangerous problem. It's 10000 times better than just "grabbing the bit" and pulling back, that will cause the situation to become even worse. The one rein is the safest way to stop a runaway horse and least harmful to the horse if done correctly.