Sidestick priority
DC-ATE,
as soon as you press the button, your stick has priority. It was used "in anger" at least once to recover from an upset after takeoff when incorrect re-wiring of a stick after maintenance caused reverse roll commands.
If you press and hold the button for 40 seconds, the priority status becomes latched, and your stick keeps priority even when you release the button. Useful, e. g., as mentioned, when the other pilot becomes incapacitated, and his arm still leans against the stick, deflecting it.
If the take-over button (which doubles as A/P-disconnect button) is not pressed, both stick inputs are summed, limited to the amount of a single stick's maximum deflection.
Bernd