Sorry been cleaning ductwork for 20 yrs and we NEVER have turned on furnace fan/ airhandler while cleaning!!
In fact the power to the furnace is shut off at breaker panel.
Tell me how you don't disturb the debris inside the duct work and then send it through supply while cleaning returns when running airhandler please.
I use negative air equipment approx 10k CFM and scorpion rod system from Heatseal .
Oh and we also clean the main ductwork which makes up 75% of the entire ductwork.
We block off Supply/ Return at the furnace filter by inserting a piece of cardboard the size of furnace filter. Common filters up here are 20x20 20x25 and 16x25.
We also block off all offshoots with cardboard and leave one open on each supply and cold for airflow.
Again tell me how you don't pull debris that you disturbed through fan and into evaporator coil / heating elements and out the supply while cleaning the dirtiest part of the ductwork ( cold air returns) Already know you don't clean the mains .
No apology needed!
Your question has merit and deserves a fantastic answer.
During the cleaning process, each supply duct is cleaned one at at time, with the vacuum box secured on the duct opening.
The vacuum is then attached to the vacuum box, pulling the debris in that direction, augmented by the air movement generated by the air handler.
Since you're cleaning one duct at a time, the disturbance (agitation) of debris is limited to that duct, not the others, so no dust/debris blows out elsewhere in the home.
It works extremely well, to which I can attest for having cleaned thousands of buildings personally, as well as hundreds of thousands of buildings cleaned by purchasers of the RamAir system.
A simple Google search of "RamAir Duct Cleaning" will reveal literally hundreds of websites of cleaners who clearly state on their site "We proudly use the RamAir system..." and such.
Oh, and we clean the main ductwork (in the industry we refer to that as the trunk line) as well.
I don't agree that the trunkline comprises 75% of the ductwork, but I'm not going to split hairs with you. lol
So before you make statements like "Already know you don't clean the mains", might wanna fact check first ; )