The ECU is a specialized small computer that interacts with and controls various vehicle sensors and solenoids. These sensors and solenoids include the temperature of the motor (Harleys head temperature sensor), outside ambient temperature and pressure (altitude), throttle position (TPS), engine speed (RPM), load (kPA or Manifold Absolute Pressure/MAP), and many more.

The ECU combines all data gathered by the above mentioned sensors and, using engine "maps" that have been flashed into its memory, makes adjustments so that the engine works as instructed. The newer model Harley Davidsons, (2008 and later), also have a narrow band O2 sensor and can run in 'closed loop'. Closed loop allows the ECU to adjust select areas of the map, which is otherwise unavailable for modification while the vehicle is running.

The Direct Link software, in conjunction with a Vehicle Communication Module (TR4 or TR4b), programs new instructions into the Harley Davidson ECU and also assists the user in determining what those new instructions should be.