When stick are centered, vehicle is locked with GPS in PositionHold mode.
If Pitch stick is moved above the deadband value, the Multirotor exit PositionHold mode and go forward using the primary stabilized mode. (eg. Attitude mode).
Now the Pitch stick returns to his center: GPS assist enter in brake mode, reduces velocity and finally return to PositionHold.
While braking, any change in throttle enables the Thrust mode defined in primary flight mode.
When in position hold, all Pitch and Roll commands are ignored until throttle is at or below "neutral" for safety reasons.
When the vehicle is in GPS assist position hold mode, you can enter the Thrust mode defined in primary flight mode by raising throttle above neutral.
Define assisted flight modes
Go to Config > Input > Flight Mode Switch Settings
In Assisted control column, the GPSAssist tag add GPS Assist to the third flight mode pos, basically Rattitude mode + CruiseControl