What is meaning of Head Reach of ships?

Head Reach is defined as a distance along the direction of the course at the moment when the   “full astern” command was given. The distance is measured from the moment when the “full astern” command is given until the vessel is stopped dead in the water.

Note:

Stopping ability is measured by the “track reach” and “head reach” realized in a stop engine-full astern maneuver performed after a steady approach at the test speed until ahead speed in ship coordinates changes sign (i.e., vessel starts going backward).