Here is a video of a man shooting himself in the chest with a 44 magnum. He follows it up by shooting a man in the chest with a 308 WIN rifle who happens to be standing on one leg. Neither man moves a muscle (they do have some serious faith in the products though). I laugh when I hear knockdown power. If a 44 magnum and a 308 WIN can't "knock down" a man, a 9mm or 40SW sure can't. Heck, even a 45 ACP might not be able to do it.
Know the status of your weapon
Keep your muzzle oriented so that no one will be hurt if the firearm discharges
Keep your finger off the trigger until you have an adequate sight picture
Maintain situational awareness