IMO, it is purely a marketing ploy by S&W to get people to buy a gun so light it couldn't possibly handle +P ammo. SAAMI is the organization, of which S&W is a member, that sets standards for ammo. It introduced +P in the mid-1970's at the same time it downgraded the pressure rating for standard ammo. +P is essentially standard pressure .38 Special before the pressure standards were revised. SAAMI has also changed upward the proof pressures for .38 Special and now referrs all testing to +P proof pressures. Gun manufacturers are party to these changes and only stupid ones would design a gun that couldn't handle any pressure commercial ammo that it could properly chamber.