This is specific to the preview version of Elite: Dangerous on Xbox One - on PC this appears to be J, but obviously that doesn't apply to the Xbox controller.
In one of the training missions you hyperspace to the next system and then use your Frame Shift Drive (FSD) to travel to a nearby station.
However, once you get there you don't seem to be able to get out of FSD, you just zoom around the destination. If you use the A+Up to exit you drop out wherever you are - many minutes or hours away at normal speeds.
When you are close to the station and travelling below C you seem to get a prompt at the bottom of the screen
SAFE DISENGAGE READY, but there's no indication or help in game as to how you action that. There doesn't appear to be a binding in any of the controls or options indicating how you carry out a safe disengage (or even what a safe disengage is).
How do I action that prompt?
A, which brings up a sub menu, then press
upon the D-pad.