The DJI ESC mounts on the underside of the prop arm, below the motor. As an electronic speed controller, it regulates how much power gets sent to the motor based on instructions received from the flight control system. With the S900, there is one ESC per motor/rotorhead. Additionally, there are two versions - a red LED version and a green LED version. It is crucial to replaced the ESC with the same version that was originally there.
Note: Installation requires soldering. Please see the S900 user manual for installation instructions.