"Nani?!" is almost always used when a character realizes their opponent is vastly more powerful than they expected.
Suddenly, a high-pitched, shimmering sound effect rang through the air. A bright red aura flared around Jiro’s body, and his eyes widened to the size of dinner plates. nani_ses_efektianime
It marks the exact moment a secret strategy or hidden ability is revealed. nani_ses_efektianime
Here is a helpful story that brings this classic anime trope to life. The Legend of the "Nani" Moment nani_ses_efektianime