The co-driver reads pace notes and calls corners; the official description says quick calls work without a microphone.
Agree on clear calls
Before a stage, make sure the navigator is ready to read and the driver can hear or see the available quick calls. Call the next turn early, then keep the message short. The game page does not provide a full symbol or call glossary.
Coordinate handbrake moments
The official description says the co-driver can pull the handbrake. Use that role deliberately with the driver rather than issuing conflicting inputs. The exact timing and effect can depend on the current game build, so learn it in a safe run.
Official source
The official Roblox listing is the source for the game features described here. See the source link below.