(After having read Piaget, I'm not sure if this could be considered pre-exercise, or not.)
Pre-exercise through symbolic assimilation could be alternately described as imitating a mode of being in behavior so as to increase the Child's understanding of said role, and to prepare them for such a role when they reach adulthood. It is pre-exercise precisely for this finalistic preparatory nature. An example; a child caring for a doll, like her mother would, is pre-exercise of the maternal instinct. Acting as such in order to assimilate to the role that she doesn't yet fully comprehend on a conceptual level.