The slip ball is more accurately described as a lateral accelerometer, thus it is a measure of aircraft lateral acceleration due to the forces acting on the aircraft (pendulum effect).
Take care not to confuse acceleration with a measure of aerodynamic sideslip as indicated by ‘a piece of string across the windshield’ in the relative airflow.
Following an engine failure it is important to use both rudder, and bank towards the live engine. Use of rudder provides a yawing moment that balances the moment from the thrust of the live engine. Using roll by banking towards the live engine reduces the aerodynamic force required from the rudder (reduced rudder foot loads) and reduces drag.
Thus for the JAA question: with wings level and only using rudder, the aircraft will have aerodynamic sideslip (piece of string over your right shoulder), but the lateral accelerometer (ball) will be centered – no slip indicated.
When correctly applying bank towards the live engine the ball is displaced a small amount (1 ball) towards the live engine i.e. in the direction of roll, and the string should be centred.
References: Extract from FAA/CAA PSM+ICR Turboprop training CD, ‘asymmetric flight’ and ‘Asymmetric flight at low airspeeds' by Tim Allen, BAE SYSTEMS safety magazine ‘JETSETS’ Issue 4; Jan 1997
I am requesting authority to attach the refs above to a future post