"Knock down power" is a myth spread over many, many decades (centuries!) of firearms use. The power of a given bullet is a function of its weight and velocity, measured in terms of its kinetic energy. If it hits something, some or all of that energy is transferred to what it hits. If it's a bullet hitting an animal (or a human being), its design will determine how much energy is transferred to its target. A full-metal-jacket, non-expanding bullet will drill a narrow hole straight through, and probably exit (except for low-power handgun rounds). A hollow- or soft-point bullet will expand, greatly increasing the speed of deceleration, and may or may not over-penetrate, depending on mass, velocity and target thickness and density.
All of the energy transferred to the target disrupts tissue and/or breaks bone. If sufficient energy is transferred to muscle tissue, a violent, spasmodic reaction can occur, which can lead to the target falling over. This is NOT the bullet "knocking down" the target, but a muscular reaction. On the other hand, if a major bone structure is broken, it may well be that the body can't remain standing due to a lack of sufficient support structure. Again, it's not the bullet knocking the target over: it's that the target is no longer able to support its own weight.
Other factors leading to the target falling over can be instant cessation of nervous system function (e.g. a brain or spine shot), or the cumulative effects of bleeding (i.e. oxygen starvation affecting the brain and/or muscles).
A sufficiently heavy "punch" from a major caliber can give the impression of knocking a target over: but this is NOT the case. If you look up the momentum (mass multiplied by velocity) of the bullet, and compare it to the momentum of the target, you'll see that the first is usually insignificant compared to the second. (I'm not talking about varmint hunting, where a .223 hollowpoint can send a groundhog flying through the air, of course! I'm talking about deer and/or human targets, which are pretty similar in weight.)
Hope this helps...