An agitator on a GBC module stirs or disrupts flow of balls to prevent jamming of the flow. Agitation is usually required whenever the ball path narrows. The feeders of some lift mechanisms provide their own agitation, as in most conveyors, and step feeders. Others, such as many ball pumps, require the addition of an agitator.
When creating an agitator, you can often just extend or tap into the motion or linkage already present in the lift mechanism. Often just a little extra bumping is sufficient.