[abdullahkhalids]

I think what happens is that E is moving the whole gear D move around. I think you realize this. In other words the motion of E does not change the teeth of E and the teeth of D which are in contact with each other.

But then the gear LeftC is making D rotate around the circumference of E i.e. changing the teeth of E and teeth of D which are in contact. Since this is in the same direction as above, you get 2x speed.

Essentially imaging you sitting on the edge of a plate. E is making the whole plate spin. And leftC is making you run around the circumference of the plate in the same direction as the direction of rotation. So your angular speed becomes twice as much as the speed of the plate.

[kd5bjo]

E is the output gear, and B is the input. The arrangement on the right is to make the frame A and gear C turn in opposite directions, and D adds the two rotations together again to turn E twice for each revolution of B.