The value depends on the rotation of the object. Upward rotation (0 degrees) is mapped to the value given by begin, and moves toward end when the object rotates clockwise, up to end after one revolution (360 degrees).
See the
animation documentation for further explanations of the animation system.
PlayAnimation("Turn", 5, Anim_R(0, GetAnimationLength("Turn")), Anim_Const(1000));
Plays the animation "Turn" in slot 5 and superimposes any other potential animations in slot 5. The animation is synchronized to the object's rotation so that the full animation is played once per revolution.