It stores your driving data on a USB stick plugged into the B&M computer.
Then after its stored some data you can plug it into your PC and it will show you details about your driving and how you can save money by driving more efficiently.
I used it for a bit but it was buggy as hell and wouldn't even read my data half the time. It also gave me 5 out of 5 for my gear changes, I knew this was ******** because i had just got the car and I was ragging it everywhere changing gears as late a physically possible