Physics tells us the impact force of the bullet can't be any more forceful than the recoil of the shot (abarring extenuating influences such as shock absorption from a semi auto mechanism).
The object of this "designed for personal defense" ammo has to do with the transfer of kinetic energy. The idea is to transfer 100% of the energy of the projectile into the body of the attacker. This is what "over penetration" is all about, and why it is so bad. If the projectile exits the other side of the attacker then by definition it did not leave all the energy within the attacker's body, which is useful deterrence left on the table, so to speak,
and a hazard to anything behind your target. The hard part to balance is that the projectile has to have enough penetration to go through clothing and skin while losing as little energy as possible BUT fragment as much as possible once into the wet interior of the subject to really do some damage. Highly frangible loads will leave more kinetic energy (and lots of shrapnel) behind but may not penetrate heavy clothing (or light armor). Heavy loads will punch through outerwear but may go through your opponent, the wall behind, and possibly one or more of your neighbors.
I'm not entirely convinced of the whole "stops your heart" thing. Any pressure wave strong enough to stop your heart would probably burst the blood vessels and dissipate long before reaching your heart. Hydrostatic shock is real, though, as evidenced by the following picture:
Note that each wound channel shows a small entry point followed by a ballooned portion where the bullet mushroomed and generated lateral force. The more lateral force generated by the expansion/fragmentation, the shorter the penetration, and the more damage/energy transferred to the target. The idea is to damage as much meat as possible in the hopes that something important will be broken in the process WITHOUT causing hazard to anything beyond your target.
Hunting bullets (the ones for hunting the seriously dangerous game) are somewhat of an exception, because they are designed to stay in one piece and penetrate through 8 feet of Cape Buffalo while "key holing" or tumbling so as to create as wide a tunnel as possible all the way through.
--Patrick
