Commissar Yarrick's powerfield forces the reroll of successful wounds but this can get wonky in the shooting phase. That's fine and dandy but here's the question, at what toughness are those rolls resolved? Is it Yarrick's stat or the dominant stat of the unit? Yarrick is T4, most guardsmen are T3, and Ogryns are T5 so it matters whether it is resolved against the dominant toughness of the unit or against Yarrick's own value.