Impact (X) "On a successful Charge, a Model with the Impact (X) Special Rule temporarily increases a Characteristic by 1."
If a Unit that includes any Models with the Impact (X) Special Rule or any Models that have any Weapons with the Impact (X) Special Rule, and that Unit makes a successful Charge, then this Special Rule is triggered. Once triggered, until the end of that Assault Phase, when making Melee Attacks for Models in that Unit, each Model or Weapon that is selected for one of those Models, that has a variant of this Special Rule, gains a modifier of +1 to any Characteristics that is the value of X.