As the prize in one of the quizzes he won, he chose an advanced college text on the atom--something far beyond his powers at the time.
from Death Be Not Proud by John Gunther
Gunther is using continual prepositional phrases leading up to the main point he is trying to make. Then, he uses a dash to go into deeper meaning and thought about the subject.
As the group of students in one of the classes had done, they chose a challenging topic of history--something more advanced than their brains could understand at that age.