A backmarker is a driver who is running at the back of the field, typically one or more laps behind the race leader. Backmarkers are usually from smaller teams with slower cars. When a leading driver catches a backmarker, the backmarker is expected to let them pass without holding them up. Officials signal this using blue flags. Managing backmarkers without losing time is an important skill for frontrunning drivers.
Example: Verstappen is catching the backmarkers now — this will test whether he can lap them without losing time.
Backmarker does not mean a driver who is simply slow on a given lap; it specifically refers to drivers who are running a full lap or more behind the leaders.
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