starting the car in gear with the clutch engaged causes it to lurch forwards or backwards, since the starter motor by itself produces sufficient torque to move the whole vehicle; this can be highly dangerous, especially if the parking brake is not firmly applied and can be injurious to the starter and drivetrain. therefore, novice drivers are taught to rock the knob of a manual gearbox from side to side before starting the engine to confirm that the gearbox is in neutral. for the same reason, modern cars require the clutch pedal to be depressed before the starter will engage. the latter practice is also useful in extremely cold conditions or with a weak battery, as it avoids the starter motor also having to turn over a gearbox full of cold and highly viscous oil.