The ECU is reset (the dealer uses either a diagnostics computer called a "Select Monitor" to zero all memory settings, or he disconnects the battery for long enough to allow the back up capacitor to drain). After this has happened, the ECU reverts to factory settings and has no previous history of limits learnt from the knock points of previous fuel grades. The car is then driven normally (ie briskly!), and the ECU starts assessing its parameters and adjusts the maximum settings accordingly, such that no engine knockhing is detected. This status will exist until either low grade/defective fuel is used or the ECU detects a fault that it must act upon.