Under certain circumstances (change of altitude, etc.) the interior of HUNTER may have a substantial pressure difference compared with the environment. This can lead to two consequences in HUNTER.
Internal overpressure: making the keys stiff and difficult to operate.
External overpressure: a tendency to ingest any rainwater adjacent to the seals, and in extreme cases, the possibility of activating the keys.
In either case, any pressure differential can be released by slightly loosening the battery cap.
If you are taking HUNTER on an airline trip, this is a sensible precaution to take.