Night. Two cars race down a mountain road. The Civic crowds out the AE86 on its tail at every turn; how will the AE86 find the space to pass? Both cars dash into a tunnel. The AE86's headlights flip down into its hood. Exiting the tunnel and drifting into the next turn, the Civic's driver searches his mirrors for the AE86, but finds nothing. He is stunned to hear its engine roaring right beside him. The AE86 emerges from the darkness, its headlights flipping back up to reveal its place on the inside track, passing. After this turn, the AE86 is in the lead. It wins.
This was the first of several instances in Initial D when Takumi deliberately shut off his headlights in order to pass his opponent. This is his "blind attack", so called because—due to the night conditions under which he races—he cannot see for the duration of the technique.
And it is a technique, at least as the manga/anime would have you believe. The purpose is twofold. First, it hides Takumi from his opponent. I believe (i.e I believe that the reader/viewer is to believe) this makes it harder for the opponent to counter Takumi's approaches, since he will have to guess where Takumi is coming from. Second, when the headlights flip down, the drag associated with them disappears, so the car goes faster. The temporary blindness during the attack, and the fact that Takumi contends with it effortlessly, are, I think, meant to signal the blind attack as another feature of Takumi's advanced and unorthodox—even idiosyncratic—driving style.
How realistic is it? I can't say for sure. I don't have the right knowledge—not of racing, not of aerodynamics—and if the ones who do have answered this question or questions adjacent, I couldn't find them.