The quick answer is that we blast walnut powder through your intake valves to clean carbon out in a much safer and better way than using chemicals and heat cycling.
Modern engines are usually direct injection, because this process allows for advantages in fuel consumption as compared to conventional fuel injection, it also gives you more power. The main disadvantage of this is that the normal bathing (and therefore cleaning) of the intake valves via the fuel they are exposed to doesn't happen anymore. Over time this will lead to deposits of carbonized fuel and oil in the intake tract, as well as on the intake valves themselves (known as carbonizing).
This isn't necessarily bad for your engine right away, but as time goes on this can definitely have a negative impact on your European vehicle's efficiency.
This blockage of deposits can stop the valves from closing properly and you may develop a bumpy idle, vibrations, and diminished throttle response. One more thing you may notice is increased oil consumption.