With traditional sweeping rods you create surface tension, and the brush grabs the surface causing the flue to move while you are trying to sweep it.
Should there be any build up in the flue this causes narrowing of the flue and even more surface tension making sweeping hard and causing movement in the flue you are sweeping. Surprisingly traditional sweeping is more dangerous to the structure of the chimney!
When power sweeping, the brush head through centrifugal force expands to suite the flue you are sweeping and it also centers itself because it is not a traditional brush so there is no surface tension being created in the sweeping process as the head adjusts to fit the flue width. We adjust the force of the sweeping action by increasing the speed of the drill and this has the effect of stiffening the brushing action ensuring you get the cleanest sweep.