Those dwarves who are tasked with checking out suspect mine passages to detect the presence of lethal gases. By tradition successful knockermen become kings of the dwarves.
The brother of Cheery Littlebottom volunteered to train for this profession, which must have the same sort of high-risk high-status appeal to young male Dwarfs as "fireman" does in our society. According to Cheery, he loved his job and had no regrets about his career choice, right up to the moment he was killed in an explosion somewhere under Borogravia.
We are informed that a visible symptom of the stresses among Dwarfdom is the way in which time-honoured practices are changing. To many Dwarfs, this is the way we have done things for thousands of years, where purely practical techniques to do with mining and living underground have remained unchanged for millenia, and which have thus acquired a patina of custom and ritual only a step away from being religious rites. Indeed, the very robes and layers of clothing worn by the most strict-observance deep-down Dwarfs and their Grags are, effectively, the uniform of the traditional knockerman. This is no accident: the layers of fabric and leather worn by the knockerman serve to provide protection from the blast and heat of a pocket of mine-damp which he has just detected, and then proceeds to detonate by sling-shooting a bundle of burning rags into. While a good throw with a sling can cover seventy to a hundred yards, it is not unknown for the blast radius of an ignited gas pocket to exceed this safety zone. If the knockerman is lucky, the worst he can expect is a backwash of hot gas. If he is unlucky, he becomes a proud family memory. As Cheery pointed out, a Dwarf family will regard having a knockerman son as a huge honour, but accepted practice is to treat him as a dead dwarf walking from Day One, as it minimises the inevitable grief.
Traditionally, a knockerman detects mine-gas by means of a cage of cricket-like insects he carries with him, which are sensitive to its presence and fall unconscious long before Dwarfs can detect the gas. However, in that greatest of Dwarf cities, Ankh-Morpork, a very clever Dwarf has devised the safety lamp, which does not ignite minedamp: instead its flame burns blue to alert the operator. This little thing has become symbolic of huge dissent, overturning, as it has, centuries or maybe millennia of established practice. Dwarfs in Copperhead and Llamedos are all for it; egged on by deep-downers, Dwarfs in Überwald consider it an abomination.
Yet the Deep-Down dwarfs aren't above new-fangled innovation where it suits them or adds an edge to intense discussions on theology and what it means to be a Dwarf. Witness the devices used in Ankh-Morpork which in some definite and terminal way, throw jets of flame and which are explained away as a different method of dealing with mine-damp.
And why do Deep-Down grags wear knockermen's costume? Just as the costume offers protection against poisonous and deadly gas in a deep mine-working, above-ground it offers symbolic protection and rejection of deadly fresh air and daylight...