The unit in the link uses two load cells, mounted to arms under the belt. One is prior to the infeed, the other is after the infeed. The incoming cell is used to tare the belt, and the outgoing one measures the actual load of material on the belt.
As the belt runs, the controller for the unit acts as an integrator, basically summing up the weight of the material passing over the second cell and coming up with a mass-flow rate in pounds per hour. The flow rate is determined by the belt speed, which is controlled by a small drive. In absolute mode, you run the belt at a constant speed and just read the total pounds/hour output. In gravimetric mode, you enter a pounds/hour setpoint, and the controller regulates the belt speed to achieve that.