if the tranny is in 1st, clutch out, and you go down hill, your coasting, rpms at like 3k, your moving slow, all of a sudden you push in clutch, truck rolls to 10mph, but the dline is still spinning your trans in 1st gear dbl low, which at 10 mph is prob 20k rpms, make sense? Its fine to coast downhill in gear if the clutch is out. Its not safe, but its ok on the drivetrain, to coast downhill with the transmission in neutral.