The mechanism guiding manual gearboxes is complex, involving interlocking teeth and shifting mechanisms. Automatic gearboxes take out the need to get a clutch pedal, which disconnects the motor from the transmission in an effort to alter gear.   The clutch equipment is provided with a single gear preset on it https://hectorwikom.thezenweb.com/new-step-by-step-map-for-automobile-gearbox-72169259