He is probably thinking of the B/UP SPD/ALT button on the NEO series. It does not affect the flight control laws in any way; however it brings the “BUSS” backup speed scale (AKA fast / slow AOA indicator) and GPS altimeter onto the PFD without requiring to turn off multiple air data sources like previous iterations. It works independently on each PFD, and since the underlying data is still correct the autopilot will still work (though the ECAM will tell you to disconnect it).
Can switch it on and off all day with no ill effect supposedly; still haven’t had the balls to try it in the air yet!