To answer your question Ian.. I think the box increases the pressure at the fuel rail to increase throttle responce and also if its turbocharged creates a delay in your pressure from the turbo being regestered just enugh to increase the psi by about 2-4psi before the wast gate opens... This cobination is what gives the extra power/torque ect and also better mpg as the engine dose not work as hard to maintain the same speed.. For example after a tuning box was fitted on a touran i had it went from 55mp to 64 mpg and i hardly ever took it out of 6th gear where as before i was always up and down the box... The torque is the real bonus for mpg .. And incresed pressure at the fuel rail gives a great throttle responce
If im wrong ooppssii