In case of sliding mesh gearbox, the output shaft gears constantly rotate While using the output shaft while in other gearbox, the output shaft gears are mounted with bearings that's why these gears rotates separately. Sensors in automatic transmissions use interior oil tension to change gears at the suitable time. https://ricardossrsr.blogscribble.com/33872736/indicators-on-gearbox-auto-you-should-know