The operator will usually cut an access hole into the duct line and connect the vacuum. Done properly, the operator will block off areas along the duct with bladders (inflatable balloon type things) to create a vacuum in sections of the duct. The vacuum then sucks debris, dust, etc back towards the Negative Air Machine. Many operators use an air compressor to help with the removal of dust & dirt. They blow air from the other end to create a "tornado" like effect. Some operators will combine the benefits of the Rotary Brush Method (mentioned on this website) with Negative Air and use a brush system to scrub the ducts clean and the Negative Air Machine to remove the dirt, dust, & debris.
Negative Air Systems are suggested on heavy commercial and industrial type jobs. These large jobs usually contain huge duct that a Rotary Brush method by itself cannot clean efficiently. The large jobs usually need a powerful vacuum (Negative Air Machine) to remove stuff out of the ducts. With this said, Negative Air method is usually not the popular method for residential & light commercial jobs. Usually, the equipment is too big and bulky for residential & light commercial jobs. Also, the rotary brush system usually does a better job than the Negative Air method used by itself on the smaller normal size duct.
Website Negative Air Vacuum Recommendation: Negative Air Vacuums
Compare Negative Air to Rotary Brush Methods: "Types of Air Duct Cleaning Equipment"