Driving in any forward gear, the clutch is depressed, shifted into neutral, clutch is released, engine is revmatched apropriatly in neutral for the desired speed, than the clutch is depressed again and shifted into the desired gear to syncronise the speed of the tires with the speed of the transmission.
common with semi-trucks with a manual transmission lacking syncronisers.
my syncros were so shot that i had to double clutch just to shift into 3rd gear.