Known as gearing, this is the mechanism by which the EU racks up its laws.
First of all, it prevents member states implementing their own controls in a vital policy area (such as immigration), and then starts to regulate in these areas itself. The regulation is invariably incomplete and functions poorly, requiring more legislation. It is then not long before there are demands for additional laws, whence the EU commission happily obliges with proposals – grandly declaring that the member states are calling for "more Europe".
Without fully realising it, that is what The Daily Mail is complaining about today, with a long lament about how police across the European Union are to be given free access to the DNA of four million Britons, millions of fingerprints, and to vehicle and driver registrations.
This is the so-called "Prum Convention " on cross-border police co-operation, which has now come into force throughout the EU, allowing for the mutual sharing of personal data held by police forces within the Community.
The Tories are also calling the convention a "sell-out", not least because this deal would have been part of the EU constitution, but has now been agreed separately as an intergovernmental agreement. London MEP Syed Kamall, for instance, says: "We are sleepwalking into a Big Brother Europe while our government stands idly by."
The problem is, though, that within the framework of our membership of the EU, the arrangement is both consistent and necessary. The Treaties oblige us to allow free movement of the citizens of EU member states across our borders; we are required to allow them to take up residence and they are allowed to bring their vehicles and drive on the licenses issued by their home countries.
Furthermore, we are not allowed to carry out any border checks and we cannot impose any conditions on entry – such as requiring evidence that individuals do not have criminal records.
The problem is, of course, that freedom of movement extends to criminals and as well as honest citizens, leaving the police in an impossible position of having to check out potential criminals – to say nothing of drivers and vehicle of EU citizens resident here - when their records are in another country.
Thus, with EU law having created the situation in the first place, and prevented member states from implementing their own controls, we are more or less forced to accept further EU-wide agreements to deal with the consequences – in this case the Prum Convention. That is engrenage in practice.
It is all very well for the Mail and the Tories to moan, therefore. But the answer to dealing with incoming criminals – without resorting to EU-wide agreements – is re-instate border controls and develop enhanced co-operation through Interpol. Neither of those options are available, however, as long as we are members of the European Union.
If the Tories and the Mail don't like the current arrangements, therefore, there is only one answer – but, needless to say – it is one they are not prepared to countenance. Instead, we get empty rhetoric, while engrenage continues to drive the process of European integration.