i was wondering if someone could tell me what that little piece of metal is that is screwes into the headstock on some guitars with the locking nut system?
i have a guitar with a floyd rose myself and am wondering if it really benefits anything or what its function is.
It does nothing to tuning stability actually. It simply allows the strings to sit flat in the nut so they don't go sharp when you tighten the nut, knocking it out of tune and causing string breaks. Less about stability as making sure they don't go out of tune in the first place.
ok, thats pretty much what i thought it was for but wasnt too sure.

thanks for clearing it up!
One small note - on some guitars the retainer bar can have a dramatic effect on tuning stability if the nut is not properly secured to the neck (by "proper" I mean through-the-neck bolts, and/or glue. Normal screws are too weak!). This happens when the missing string retainer causes the string pull to actually exert an almost upwards force on the nut, in other words lifting it upwards.

This happened on a Peavey I have where the bar was missing when I got it. I didn't worry about it until the guitar started behaving oddly and exhibited very strange tuning problems. It turned out every time I did a dive bomb or pullup, the string tension would pull the nut back and forth (just a fraction of a millimeter), loosening the screws that were supposed to hold it.

It was a days work to rebuild with epoxy and glue the nut back on (in addition to the screws). After that + a new retainer bar the guitar is rock solid again.

So keep in mind: never ever remove the retainer bar, the string pressure is very important to keep the nut in position!