Anyways, how does it work amd is it worth getting one?
Current Rig:

Ibanez GRG270dx
DOD 40watt amp (don't know the model)
Dunlop Crybaby (moded)

I know it's sad but it'll do for now.

Member of the Nasi Lemak Appreciation Club!
I think its a switch you install on the low E string. When its fastened down, its a regular E String, but when you flick it or something it downtunes the Low E string to a D.

It just allows you to go to drop D and back faster.
Been away, am back
Allows you to drop tune to drop-D without messing with the tuners at the headstock. Usually recommended for guitars with double locking system and with Floyd Rose tremolo system.

D-Tuner on Ibanez ZR system

D-Tuner on Ibanez fixed Ax series

"Play with your ears" - Yngwie Malmsteen, Paul Gilbert
Thats what she said...
It was EvH who either invented it or made it popular. Not sure. But it only works on one-way OFR's. The "D-Tuna" that is. Not the other kinds (shown above).