What is variable speed transmission？

What is the variable speed transmission？An automobile requires high torque when climbing hills and when starting, even though they are performed at low speeds. On other hand, when running at high speeds on level roads, high torque is not required because of momentum.  So requirement of a device is occur, which can change the vehicle’s torque and its speed according to road condition or when the driver need. This device is known as power gearbox.