coin drop

A coin drop is the moment when something suddenly clicks into understanding—like hearing a coin fall through a mechanism before it registers.

The phrase comes from arcade games, vending machines, and pay phones, where inserting a coin produced an audible drop before the machine responded.

There was a coin drop moment when I finally understood what she'd been trying to tell me.

The term also refers literally to charity events where coins are collected: The school's annual coin drop raised three thousand dollars.