Hi Piers, Colin Belton will give you technical bits but basically a live remap is carried out on a rolling road,
its a fine tuning of the program in the ECU ( Engine Control Unit ) that when carried out should give you 10 - 15%
improvement of the BHP and Torque, end result is more power and improved acceleration throughout the rev range,
it's well worth having done, the price around £500 ish, considering the prices of exhaust systems with dubious power
claims, I think a monkey is a reasonable price to pay for the work, which normally takes about 4 hours.
hope this helps.