The Downshifter D-Tuner is a mechanism that allows the player to easily drop the tune of a string. It is made for Ibanez and is available in at least two versions: one for a fixed bridge and a second for the ZR tremolo bridge. It installed on several Ibanez models, including the GAX75, AX125, S1625 and S1675FM.
The D-Tuner is preset to drop the pitch one whole step to allow the player to switch to drop D tuning on the fly without the need for fine tuning. However the amount of drop is adjustable and it can even be used essentially in reverse to raise the tuning of the string. It is similar in function to an EVH D-Tuna.