Just as not all warriors are not equal in their might and resolve, so too are some fleeter of foot (or hoof).
When charging, units entirely made of models with the Swiftstride special rule roll 3D6, discard the lowest result, and add the result to their Move value.
For example: A unit of Harpies charge and roll 3D6, with results 2, 4 and 5. Thus they charge 4 (their Movement) + 4 + 5 = 13".
When fleeing and pursuing, units entirely made of models with the Swiftstride special rule roll 3D6 and discard the lowest result.
For example: Dark Riders flee and roll 3D6, with results 1, 3 and 5. They can therefore flee: 3 + 5 = 8".