It means that originally, you only have 10% lifesteal from the item's base stats. However, if the item passive is on, you get 25% lifesteal instead. After dealing 600-1200 damage (based on level of hero) within the 40 seconds, it will remove the 25% lifesteal, and only gives you the base stat of 10% lifesteal.
Since the damage dealt for the 25% lifesteal regens over 40 seconds, it allows you to have 25% lifesteal (instead of 10%) for 15-30 damage dealt per second (based on level).
Posted: Oct 28, 2016