The red arrow is pointing to the rev limiter relay. A surprisingly obscure component. It isn't mentioned in the factory manual or the Bentley, only the Porsche 1978 Service Information brochure.
It's obscure until it fails, because it typically leads to intermittent engine cutouts that can be maddening to track down. It works by taking pulses off the tach, and at redline it cuts power to the fuel pump.
You can disable it by pulling the white connector. The car still runs, but you lose the rev limiter.
Mark