Your mind races, "What is this? Who, what's happening?" In retrospect all of these questions will seem silly - you're being mugged; some thug wants your money and he's used the element of surprise with violence and the threat of more violence to scare you into compliance.
But in the moment, time simultaneously races and stands still. Do you fight back? If so how? He's still screaming, and he's starting to slap you again, and again. You tell him to stop - you're frightened, if you give him the money, he'll stop... right?
The truth is, maybe he will and maybe he won't. There are statistics to support either argument. Life and safety should be your primary concern. Not the money, the groceries, or even the car. You want to get to safety as fast as possible. This is the situation you have been training for, for several years.
You know how to execute an inside-to-outside block, a palm punch, an elbow strike, or Il Gi Konk Kyuck (finger strike) to the eye. You have practiced these techniques a thousand times enroute to getting your Red Belt. You know how to turn to the right or left and strike with Dwi Cha Gi (back kick) followed by a brisk run (you do know how to run, right?).
But, you're frozen in place. Why, because you're thinking. He used surprise to catch you off gaurd and instead of letting your instincts, training and muscle memory take over - you hesitated. You have only one real decision left - do you react as you've trained, or not. There is no middle ground; you cannot half hit back. You cannot slap him and walk away.
If you have been using Karate class as a social endeavor, time away from the job, the kids, the pressures of life, then hope he takes your money and leaves, or someone sees you and calls the police.
But if you have used each and every class to hone you skills through repetition, repetition, and repetition; if you have been executing your kicks with hip extensions, your palm punches and elbow strikes with conviction, striking beyond the point of impact - if this is the way you have trained - then trust your instincts.
I don't mean to make light of a terrible situation, but in the words of Yoda, the great Jedi Master, Do or do not, there is no try. Trust your instincts and react - or don't, but there is no half way. Practice like you need it and if you need it, you'll use it as you practiced.