Complex sentence: The leader was selected by a system in which each of the party's riding associations was allocated 100 points, which were allocated among candidates in proportion to the votes that he or she received.
Split into simpler sentences: The leader was selected by a system in which each of the party's riding associations was allocated up to 100 points. In associations in which 100 or more votes were cast, 100 points were allocated among candidates in proportion to the votes that he or she received.