low speed in either range puts the belt deeper into the sheaves don. the gear selection does not. That being said I prefer to stay in high range but towards the bottom end of the available speed. I watched to many guys unplug their 'bine last fall after listening to the expert from deere tell them to put the rotor in low. then they tried going as fast as they could and the rotor got constipated and they unplugged it on straw at a time. I always like to be able to open the concaves and shift to low range as I hate pulling straws out.
Well, deeper into the DRIVE sheave which reduces contact area as well as reduces spring tension on the DRIVEN sheave, making the belt looser, those two things combine result in a drive far more likely to slip.
Low range on the gear case puts the belt deeper into the DRIVEN sheave at the SAME rotor speed.
There, are you happy now?