The operation of an automatic transmission is explained here with help of animation. Allison-1000 transmission model, which has 6 speed and reverse is used for this purpose. The video starts with an explanation of planetary gear set. Just by engaging few clutch packs different output speed can be achieved in automatic transmission. A brief introduction of the torque converter is also given here.
Here is the 2D version of the video. In my opinion this video is more helpful than the 3D video to understand the technology. https://youtu.be/Ugao6jTyM7k