The "slot machine" term derives from the slots on the machine for inserting and retrieving coins. "Fruit machine" comes from the traditional fruit

on the spinning reels such as lemons and cherries.

Subject and Verb Slots. A sentence has two primary slots for inserting content: the subject slot and the verb slot. Readers naturally focus on the words in these slots and, rightfully, sense that the main information will be in them. [subject slot] + [verb slot]