This skill is a Special Rule that is only available to specific Star Players. Please visit this page to find a list of current and past Blood Bowl Star Players.
Savage Mauling
Once per game, when Wilhelm makes an Injury roll against an opposing player, he may choose to re-roll the result.
This reminds me of Blood Bowl back in the 90s where you could use team rerolls on armour and injuries. Removing opposing players from the pitch is always going to make life easier for you and this special rule helps with that. The only real question has to be when to use it, do you go for it at the first opportunity, or try and save it to remove a better player from the other team. The risk with waiting is that you might not break armour with Wilhelm for the rest of the game. So consider how much of the blitzing you are going to do with him, or how likely it is that opposing players might start your team turns in base contact.