The belt channel(帶脈): as its name says, it is the belt. As with most channels, different practices and/or different practitioners defined it differently. Most Neidan (Taoist yoga or Taoist meditation) practitioners focus mostly on the Ren and Du channels (任督二脈). Zhao Bichen (author of Taoist Yoga) wrote a chapter (three) specifically on the eight channels, and stressed the importance of opening all eight channels.
What do you mean by a channel being "opened"? Tough question. Take the example of water channels. A dripping pipe is certainly quite different from a pressurized water stream from a fire-fighter's hose. The concept is the same for psychic channel opening.
Martial artists put a strong emphasis on strengthening one's belt channel. And for them, it would be a strong belt with some width, certainly wider than normal belts and fitted in higher than normal belts. Physiologically speaking, the focus is on strengthening one's diaphragm muscles (rather than the abdominal muscles). With a firm belt, one's torso will be structurally strong, making the body more prepared for combat.
In chi practice, it is always a good idea to have stronger and more controlled chi. In opening up one's belt channel, the martial artist's approach is therefore recommended.