BMX trick to check mileage using key and brake

I saw this happening at a car sale (it was a flea market for cars).
some guy claimed he used to work for BMW as an mechanic, and he was
interested in buying a second hand BMW 3-series.
The mileage counter said 240.000 km, but he got in, stuck the key in to
the ignition (not completely), pushed the brake, pushed some buttons
(didn't see which ones) and suddenly the counter gave a whole other
mileage: 370.00 km!!

He said the owner (who had bought the car second hand at 40.000 km) was
probably ripped off himself when he bought it (I know the owner, he
didn't alter anything himself)
The card was second hand sold by a recognized BMW center (they had
their license revoked some 2 years ago, for fraud) with 40.000 km, but
in reality, it already had 170.00 km on it, according to this
ex-mechanic

Has anyone seen this before? What's the exact trick to get the "real"
mileage of a BMW popping up?