It has to do with train box cars and switching them between trains. They uncouple a car or a few and send them rolling through a switch to a different train. It can get pretty harsh. No idea why they call it humping.
So a conductor stands on top of the hump, uncouples the car/cars and the switchman and brakeman are running between switches sorting cars as they free roll down the hill. Most hump yards are automated now though with just a conductor on top and automatic switches.




