There's a whole series of tests done on Mythbusters that debunked the concept.
So these bullets are going EVEN SLOWER than normal ones, and it's supposed to "knock down" a person?
Think about this: a typical 9mm round will be 115 grains or 124 grains in weight. One grain is 1/7000 of a pound. We're talking about a projectile going in the ballpark of 1000 fps that weighs only about 0.0177 pounds for the 124 grain bullet. There simply is no knock-down potential.
Now, on the other hand, if you are seriously injured by one of those projectiles, you'll probably fall over in pain. Or you might also just fall over dead.