The EVH D-Tuna is a unique patented device that enables players to drop the E to D and back, in an instant. D-Tuna has been standard equipment on Eddie Van Halen's personal guitars as well as signature series locking tremolo-equipped guitars since the early 90's. Since then thousands of D-Tuna's have been sold all over the world.
The D-Tuna was designed to adapt to a double-locking tremolo, giving you the option of "drop D" tuning without unclamping the nut.
To still Float the Bridge( to a degree) you will need to stabilise the bridge with a product such as the Goldo Back Box Tremolo Stabiliser. Also available at Guitar Crazy