The cycle works like this:
1: The Tungsten atoms evaporate from the hot filament and diffuse toward the cooler bulb wall.
2: The Tungsten and halogen atoms combine on or near the bulb-wall to form tungsten halide molecules (Bromine is the most common halogen).
3: Tungsten halides remain in a vapor phase at the bulb-wall temperatures and this vapor moves toward the hot filament. A combination of diffusion and convection currents are responsible for the movement.
4: High temperatures near the filament break the tungsten halide molecules apart. The halogen atoms move back toward the bulb wall and the tungsten atoms are re-deposited on the filament. The cycle then repeats.
The The Tungsten Halogen Cycle is explained in a Video by The Edison TechCenter which provides excellent videos on all things technical.