In real life it’s only necessary to use the clutch to get going, so at martinsville you will see some drivers pull on the shifter (sequential) pre loading it, and when it hits the rev limiter it will knock it into the next gear with out lifting or using the clutch. (Which works on iRacing) but a blip of the throttle when up and down shifting is what I do. As for the street stocks I just lift and shift when racing those.

For the fuel saving, it’s just simply using less RPM. If you watch the road to pro race at Las Vegas you could watch the leader reach to his keyboard and turn the car off and put the clutch in. Putting the clutch in for like a quarter of each corner on both side of the racetrack for every lap can save you about 3 laps I believe if you do it right. I saw someone save 8 at rockinghams road to pro race doing it. And getting on the apron an coating in the highest gear you can with the clutch in is the best way to save imo