On original cars the curved green tach lines were to signal when it was ok to " floor it ". Porsche advised against W O T at low engine speeds. The green stripes on the speedo indicated at a glance acceptable city speeds. The "normal" motors had green rpm ranges from 2500 to 5000 . The "super" motors had green ranges from 3000 to 5500 and the carrera motors were to be much more tightly wound , 4000 to 7500rpm . You could feel the motors come alive where the green lines started.
There is a story about a Doctor going racing in his new a 4 cam carrera and lugging the motor around the track. He made one lap and destroyed the motor . Boohoo!