OK. So what is the thing that goes up?
Ha! I thought everyone knew about the bit of gun control folklore about the "shoulder thingy that goes up." The short (so I don't make myself late for work answer) is that the buttplate has an extra "layer" on a hinge that flips up. It is a left over from the m14 machine gun days that helped your shoulder lock into the rifle to better control full auto fire, apparently especially when shooting prone. Why semi auto M1A's have it is a bit of a mystery (tradition, I guess), but it has gotten a reputation amongst gun grabbers as something that makes them extra killier.
Seriously, google, "rifle shoulder thing that goes up"
Ironically, although the semiauto M1A like mine is banned by FSA2013, I think you can still buy a full auto M14 (that looks and works the same...just with the happys switch)...if you've got the $$$. In that case, you just might have a reason to deploy the shoulder thing that goes up.Happy switch?
MD laws...