A homeowner in Boise asked me why I bang on the flue before sweeping, and her reason for asking stuck with me
She said her late husband did the same thing on their furnace for 40 years, and it turned out he was just knocking loose soot so it would not fall in his coffee, which is not why I do it but now I think about him every time I pull out the rods.
The part about knocking loose soot so it wouldn't fall in his coffee got a laugh out of me, because I read a while back that old chimney sweeps used to bang the flue for a pretty different reason. The idea was that the vibration shakes creosote and loose junk off the walls up higher so it drops down where you can actually get it with the brush. Some guys also did it to scare birds or squirrels out before they shoved a rod up there, which makes sense if you have ever had one come flying down at your face. I get why the coffee thing stuck with her though, since 40 years of a small habit turns into a memory whether the reason was right or not. Funny how the same motion can mean something totally different to two people doing it.