Light movers are mechanical devices that, as the name implies, move an HID in a fixed pattern over a garden. A light mover can increase the coverage of a light by 140%. The real advantage is MUCH more light is available to the plants when the light is in motion. With a stationary light all that is shaded will remain in the shade, but with a mover the light will hit from different angles giving better coverage.
Light movers are found in 2 configurations: spinning arms (kind of like a ceiling fan) and along a rail that moves back and forth. The rail version seems to be much more popular with Overgrowers, probably because it will accommodate an air cooled fixture.
Light rails come in a variety of lengths, but most can be cut to fit any application. Most growers prefer a "smart rail", that is, one that pauses at each end of the rail for a period.
The biggest problem with air cooling a light on a mover seems to be dealing with air duct slack. Apparently, if the light is close to the tops of the plants, the duct may be dragged through the plants, thereby damaging them. Overgrow poster Ferret devised a solution: Use a curtain track, not a rod but the track that has all the little hangers inside that slides very easily. I screwed this to the ceiling of room and used light weight string to attach the air pipe to the little hangers in the track.
As the light moved the track hangers moved along with the air pipe stopping the air ducting from dragging over your plant tops doing damage.
The track cost about $15-$20 at any hardware or store. Simple to set up and it works .