Refer to figures.
The slip indicator consists of a ball immersed in a dampening liquid within a curved glass tube behind a window. The ball, just like the aircraft, is subject to a centrifugal force. Due to this force, the ball is pulled away from the centre of the turn.

When the aircraft is level of when the turn is coordinated, the ball lies between the lines to indicate that the apparent vertical is parallel to the aircraft axis and an equilibrium exists: the resultant of the ball weight and of the centrifugal force the ball is subject to is equal to the reaction of the base of the curved tube on the ball.