Cross-drive steering transmission
A cross-drive steering transmission is a transmission, used in tracked vehicles to allow precise and energy-efficient steering. It consists of the following main parts: two identical single-stage planetary gearings, a differential, a hydraulic pump connected to the engine (similar to an oil pump of cars), a hydraulic motor powered by the hydraulic pump, hydraulic control valves.
Source: Wikipedia — Cross-drive steering transmission (CC BY-SA 4.0)