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.
Old ProOne per game, Helmut may use his Pro skill to re-roll a single dice rolled as part of an Armour roll.
Chainsaw players don’t usually live a long life on the pitch, either by getting injured themselves or being sent off. Typically you are going to be targeting one of the better players on the opposing team, so I would probably look to use it at the first opportunity where you don’t break armour. Assuming you haven’t already used the Pro skill that turn as you can’t use it twice.