All engines produce carbon as a side effect of the engine combustion process.
Unfortunately not all of the carbon gets emitted via the exhaust system (to be filtered by the DPF or CAT), with modern recircularisation systems carbon deposits build up in the intake system.
Engine decarbonisation is a vital service to prevent bigger problems developing.
The machine used is effectively a sand blaster with walnut beads rather than sand
The process is actually rather messy, probably not a home mechanic task in reality
It is astonishing just how effective the walnut blasting is, it really does look like new
The dealerships (in America & Germany at least) recommend this process as part of a regular maintenance schedule