This is main leaf spring that you are loosening. It will lesson the d/a pull and reduce the power in which the firing pin hits the primer.
S/W does not consider this a trigger adjustment and says that this screw should be turned in all the way. You can seriously reduce the reliability of your gun. Some people do this, but I caution you that the screw will not stay put once it is loose and can loosen further by itself.
Incidently all K, L, and N frame S/W have this "feature"