POWER!
yo but this might have actually just been them blowing out the stacks, which is important in preventing stack fires, which while usually harmless can pose issues.
diesel doesn’t require a spark to ignite, just heat(that’s why diesel cars, unlike petrol cars, don’t have spark plugs), but a lot of diesel engines, particularly older ones, aren’t terribly efficient at burning their fuel. So you get some residue which builds up in the stacks, and then if you throttle up, you send a rush of very hot air into the stacks, which can ignite the diesel residue, which will then happily burn the rest of the crud that builds up in the stacks.
Waiting for Winter 