Petrol-leg?

I'm reading a book by a British author right now, and I just came across the term "petrol-leg" in the book. From context, it seems to be part of a car, and apparently lifting one will make a car slow down. Anyone familiar with this term? Thanks in advance.

ETA: A couple of sentences from the book: "Lou lifts his petrol-leg a little. The engine drops a pitch as the car slows down."