A 'Trepanner' as used in this context is a powerful coal cutting machine, resembling a huge chain saw which hauls itself along the coal face on a chain. The trepanner 'undercuts' the coal seam, which is then brought down by explosives, laid by the 'shotman'. The trepanner can by steered to ensure it remains at the base of the undulating coal seam - the man in question was carrying out this task, often in thick coal dust and the ever present danger of falling roofs.
Once the coal was brought down, conveyor belts or endless chain belts were erected and the coal shovelled on and then away 'outbye'

this system has long been superseded by armoured conveyors and coal shearing machines - like huge bacon slicers.

Robert Staines ex Warwickshire Collier