This is an option that either fleet can take after the battlefield is set up, but before the fleets are placed on the table. Either fleet commander can make this choice; if both do, then the battle simply doesn't happen; no renown is gained or lost, and neither commander can make Appeals (both can generate and use Repair points, though.)
If one commander elects to Evade and the other does not, the Evading commander must pass a Leadership test (using his Fleet Commander's leadership, if present, or else the highest Leadership in the fleet). The commander doesn't just roll 2d6; instead, roll a number of dice equal to your fleet's Attack Rating. (Rerolls can be used for this check, though if used here they are not available for the battle.) If you successfully Evade, then you are considered to have lost the battle, and your opponent has won (with the appropriate
You may not Evade if the battle is in your home sector (or at your Pirate Base.)
(The purpose of this house rule is to give the Eldar an chance to run away if they end up fighting a battle in a flare zone, which can really screw them over.)