Hit 'em in the head with something before running. I think we've discussed this before. Nothing is guaranteed to work as the first response. But if you bash the individual with a chair, rock, garbage can, you can probably get away, or effect a disarm. I'm a firm believer and practitioner in throwing something at the face. Every time I've done it, in practice, it elicits an instinctive flinch response. Hands come up to protect the face, whether the person is armed or not. Perfect for a follow up move like a chair bash, front snap kick, running, drawing a pistol, so on. Hardened individuals will see this for what it is, a distraction. The average scumbag will react to protect his eyes.
Having said that, there appears to be an instinctive response to size, both on the part of the attacker and defender. The untrained and unhardened criminals can be intimidated by size and agression, the same way as untrained, law-abiding citizens. It's human nature. And, as whitewolf's story illustrates, buddies may not be so quick to jump in, even if the defender is outnumbered. Just like when a citizen gets bullied and attacked and bystanders don't want to help. Do I have experience with this? No, only my observations of simple human nature over my few years.
Kent