C+Star
My Critics is not for you but for Airbus.
What I said is
"When yellow index it is not a Sideslip indicator, but when blue yes it is"
It is contradicting with Airbus simple explanations.
When Airbus is trying to simplify the subjects, it is ignoring some flight mechanic facts. Maybe practically it is supposed to be so. It is ok. I guess they do not want to go so much details from the perspective of pilot.
If you read Page 68 of A 330 (not A 320)FLIGHT DECK AND SYSTEM BRIEFING (AN AIRBUS DOC, You can download), they explain the functions a little bit clearer.
P.S. IIRC from aerodynamics classes, you can't have "zero sideslip" AND wings level in asymmetric flight. Something has to counter the rudder force...
That is exactly what I want to say, "airplane should bank for zerosideslip". If indexes were behaving like ball, they wouldn`t be centerlized, and pilot couldn`t understand when the side slip is zero. Therefore at given assymetric thrust condition and configuration, it automatically starts to display as a function of sideslip amount.