There are a lot of tools for agitating debris inside the ducts:
There are the old faith full tools known as a reverse skip line, forward skip line and a blow gun, these 3 tools are must haves, you simply can do duct cleaning with out them. Then there are the more advanced cleaning tools, the brush systems and the whip systems. They all have there place in the field of duct cleaning.
Brush systems work well and are excellent for taking off crusted caked on debris inside the ducting, whip systems work well for caked on debris as well as moving heavy objects downstream to the point of suction. The main point to make about advanced agitation tools is they are not time savers. Using these tools does increase the quality of the job you just need to make sure you are marketing your business in the right way so customers know there getting a better quality job and thus the reason for a more expensive cleaning.