Thanks for the technical reply RSPilot. I've translated via Babelfish what the German Ebay offer claims as follows:-
"Owing to modern technology the diesel engine is today a trend-setting drive with many advantages opposite the petrol engine. By turbochargers and high performance fuel injection systems (e.g.: Common Rail) it has enormous power reserves, which tuner uses itself gladly too makes. Opposite many other boxes that has amh booster 2 tax module a substantial advantage: Most Tuningboxen in the trade is installed before the Diesel pump and to catch so the signal, which is given to the Diesel pump, off and continues to send another signal to the Diesel pump. Simplified the Diesel pump instructed more Diesel is expressed to inject and the injection pressure increase! Thereby an increased output is reached, however any harmony is missing and the turbochargers continues to run on the old calculated values. More badly still it is that many of these boxes change the values and the amount of filling and filler times in such a way also with partial load and full load that a damage is inevitably the result! More achievement at a high price! We know it all - the smoking Diesel box drivers, who pull a black cloud behind itself, so that one is forced to switch the nebula light on. The amh booster 2 "works" there exactly in the other direction. It interrupts not the signal of the controller, but sends data to the controller. Owing to the modern fuel injection system Common Rail we developed the possibility of getting over this plant directly access to the controller. The Commonrail plant is constantly controlled by the controller. The pressure, which prevails in the plant, is constantly sent to the controller. Here the amh booster 2, which is installed directly to the Commonrail plant, is used. This sends the data now directly to the controller. This constantly adapted to the respective driving conditions, the number of revolutions and the driven course. The controller increased to instruction amh boosters 2 the pressure in Commonrail injection and fits all values optimally on (load pressure, quantity, filler time etc..)! Thus the diesel engine gets further an optimal mixture - calculates from the controller! Apply further all parameters, which the vehicle manufacturer planned as maximum load of the engine - they are not gone around by the amh booster 2. The large Clou is that the driver can determine the desired achievement by adjusting at the provided potentiometer."
How does that lot tie in with your explanation. Diesel technology is pretty new to me.