Reaching zero is easy.
Grab a Battleship, and throw an MJD on it. Undock from one of those downward facing stations and warp to an insta. Wait for your ship to drop out of warp and level out (which it won’t be level because space is water and your nose will float). Then hit the MJD and see where you go.
Your last direction of movement was down, but your ship is facing forward. But I’ve had it leap back up diagonally toward the station before. Because the game can’t decide what to call “straight ahead” anymore.