Comparison of Technologies: Ink versus Laser: Features: Duplex Unit
Printing double-page can save you a lot of money. So it´s convenient, if your printer has a duplex unit, which automatically turns the sheets. It´s just one click in the driver.
For manual bothsided printing you first print the front side, take the paper off the paper tray, fan it out properly, put it into the paper tray again to print the back side, and make sure to insert it the right way round.
There is no duplex unit in Brother´s HL-2150N. The driver advises you, how to do it manually. Due to ink technology HP´s printer pause while printing duplex, to let the ink dry before the back side is printed. Otherwise the draw-in rollers would smear.