I think I understand what you're saying because I encountered something similar in my story. At the end of book 1, I set up the plot for book 2 with a cliffhanger, and that included the trigger, the thing that causes book 2 to happen.

It doesn't matter when the trigger happens - end of book 1, beginning of book 2, or ten years ago before the chronology of the books begin. You just need to know and understand what the trigger IS. What's the thing that causes book 2 to happen? If you are clear on that, you've completed the assignment.

Cheers,
Michelle