How does a bicycle friction shifter work to change gears smoothly and efficiently?

A bicycle friction shifter works by adjusting the tension on the derailleur cable, which moves the chain between different gears on the bike's cassette or chainrings. By turning the shifter, the rider can increase or decrease the tension on the cable, causing the derailleur to move the chain to a different gear. This allows the rider to change gears smoothly and efficiently while riding.