Obvious question: which trainer do you have?

Some dumb trainers aren't adjustable for difficulty. (No, that knob should be tightened to prevent the tire from slipping. It does not increase resistance.) Some dumb trainers are adjustable. With a smart trainer wouldn't it be best to just leave it alone and let Zwift control it? Climb up the volcano, scream down the other side? That's the appeal of a smart trainer and Zwift. If you want hard all the time, or ERG mode, it's be better to just use trainer road workouts or watch a movie or something instead of playing Zwift.

Edit: never mind. just realized you were asking about the in-game setting. Not a setting on the trainer. Duh. I leave that slider at default (in the middle).