In case of sliding mesh gearbox, the output shaft gears constantly rotate With all the output shaft while in other gearbox, the output shaft gears are mounted with bearings that's why these gears rotates individually. Sensors in automatic transmissions use inner oil pressure to change gears at the appropriate time. https://gearbox-auto26036.bloggosite.com/40677383/5-simple-techniques-for-gearbox-auto