Hydrostatic Shock kind of sounds like some fancy marketing slogan to me.
I have heard the theory before and many swear by it including a guy named Roy Weatherby.
I have also heard that there is nothing to this theory and that animals the size of deer cannot be "shocked" with even the fastest and most explosive bullet a hunter would care to throw at a deer.
I have my own case in point. I shot a smallish button buck at about 40 yards with a very fast moving rather frangible 150 grn .30 caliber bullet. The deer simply stood there for a few seconds and nearly went back to eating....thought better of it and ran about 150 yards before I heard him flop over. When I found the deer...the damage to the lungs was massive. The exit hole was about the size of a large fist. The lungs looked like they had been pureed.
150 grn bullet hitting this deer at about 3,100 fps (range was about 40 yards) The deer weighed about 100 lbs (my first deer) and yet ran over a football field before piling up dead.
Hydrostatic shock? Not sure if I buy it.
