Movement is measured using a tape measure (or other measuring devices).
If a model must move Toward another game element (such as another model or Terrain piece), then it cannot increase its distance from that element for any part of its movement.
If it must move Directly Toward a game element, then it can only move in a straight line drawn from the centre of its base to the centre of the game element.
Similarly, a model which must move Away cannot decrease its distance from that element for any part of its movement.
If it must move Directly Away from a game element, then it can only move in a straight line drawn from the centre of its base away from the centre of the game element.
An Activating Model can only enter BtB with one Enemy model per activation.