Minecraft wiki entry for Bad_Luck, associated text: Bad Luck subtracts the effect level from the attribute generic.luck, making loot table entries with a high quality score less likely, and entries with negative quality more likely. It also makes the chance of bonus_rolls occurring less likely. Fishing is the only mechanism that uses quality. The new weight of each entry is floor(base_weight + quality × generic.luck). Negative levels increase luck, similar to status effect Luck.