Swift of Action "Models subject to this benefit have their Initiative modified by +1 while making Set-up Moves or Rushing."
If the White Scars Legion was selected for this Army’s Shattered Legions Trait, when a Unit that only includes Models subject to the Mutable Tactics Special Rule makes a Set-up Move or is selected to Rush, modify the Initiative Characteristic of Models in that Unit by +1 until the end of that Sub-Phase.