Knockdown power is a misused term, but it has a basis in fact.
A hard hitting slugger can knock an opponent down and out if he connects at the right spot, jaw, solar plexus, etc, while a wimpy kid could flail away all day long and not do any damage.
I've brought one up from the floor and connected under the chin to bring my opponent off his feet and knock him backwards over an office desk. No harm was done to my fist and I wasn't thrown backwards by any recoil effect.
Bullets impart energy to a small surface area and further on into flesh or bone. The amount of energy necessary to put someone off their feet is not that great.
You aren't spreading that energy over any large area.
You won't see people thrown twenty feet through a window as in a TV cop show, but they may be knocked back far enough to over balance and fall, especially if the shock disrupts the nervous system to prevent any attempt at regaining balance.
We've all seen tests where a man wearing a vest takes a hit from a .44 magnum and barely reacts. I've also seen a test where a reporter volunteered to wear the vest and the shooter suprized him by firing a 38 to the mid section while he was still talking. The reporter ended up staggering back several steps and falling on his kester cussing and squaling like a child.
So knockdown power is a factor, though not well understood as to exactly what it means.