"Sanding 'er out". Oil soot builds up in the flues and is cleaned out by generating lots of draft and air flow through the firebox and flues, done with plenty of throttle, which generates stack exhuast. Best done with a loaded train, here Norm set the brakes a bit to generate draft, at which point Vance dumps a bucket of sand into the fierbox. The massive suction of the draft turns the airborne sand into a quick sand blast inside the flues, and knocks out the soot.