Everytime you kill a player on the Strength SMP, you gain 1 permanent attack damage. Every time you die however, you lose one. The more kills you get, the more damage you will eventually do.

If you die too many times, you will start to do less and less damage until eventually you do none at all, so be careful. If you do get to the point where you do no damage, however, it's okay because you can craft strength books to redeem back up to the positives.