The attacking force allows for the element of surprise, typically the brain takes 2 seconds to acknowledge the threat and begin reactionary measures. Knife guy already knew his intentions. Knowing I'm being attacked with a knife in the video, distance is your friend. If trained and great reaction skills one could have stepped into this and disarmed or crushed his throat. Rule no 1 in a knife fight is Knowing you're going to be cut. Same with a dog, the dog is going to bite you. A dog will bite the first thing it can, Knowing this you train to attack back, go on the offensive.