The aha moment is the specific action or experience where a new user thinks 'I get it, this is valuable.' Finding and optimizing for the aha moment is the core of activation.
Analyze retention by early actions. What did retained users do in week 1 that churned users didn't?
Design onboarding to get new users to that action as fast as possible.
Making the aha moment too hard to reach. Shorten the path relentlessly.