Groo here
The main reason for Hp bullets is to reduce the chance of pass through.
Knock down or Slap[Keith term] is due to impact and pain,both
related to the size and shape of the bullet when it first hits the skin..
The damage caused deeper [by an expanding bullet] will cause more bleeding and reduce blood pressure causing one to passout.
But this takes time [ 10 to 30 sec]
In most autos , Hardaball feeds the best and that is what is wanted.
Hp will be less likely to go through or bounce and is though to be better for SD..
Find what works best in your gun then what fits your situation..
This is where revolvers manual feed is ahead.
Ps the 357 125gr full power load [1400 from4in]
Will tear large holes in skin [ starts to expand on anything ]
and maybe why It workes so well...