Watch: Untrained Knife vs Multiple Attackers
Take a look at this video from South Africa where we see one lone defender armed with a knife successfully driving off an attack by multiple opponents.
What's interesting is our defender is obviously not trained.
Yet he prevails because he is armed and he is aggressive enough to deter the attackers. This shows the power of being armed.
One interesting note about using a reverse grip can be seen at timestamp 00:06. You can see the defender's downward strike comes perilously close to stabbing his own thigh.
He then changes his grip to a regular (or hammer) grip and drives off the last attacker. He stumbles and falls to his knees as he is chasing the last attacker off at 00:11. This doesn't seem to be uncommon to see defenders overstressed during critical incidents tripping, falling, or stumbling.
This defender does a great job even though we surmise that he is untrained. Imagine how much better your reaction and your defenses will be if you get training in self-defense and defensive tactics BEFORE you need them.