Text (SM+errata): At the beginning of the end step, each opponent loses life equal to the life he or she lost this turn. (Damage causes loss of life.) [Oracle 2009/07/08]
It checks if life was lost. If life was also gained this turn, it will still trigger. For example, if 4 life was lost and 6 gained, it will see that 4 was lost and act on that. [Shadowmoor FAQ 2008/04/21]
It checks at the end of every turn, not just your turns. But it always refers to your opponents, not the player whose turn it is. [Shadowmoor FAQ 2008/04/21]
It will take effect even if it wasn't in play when the life was lost. As long as it is in play at the end of turn. [Shadowmoor FAQ 2008/04/21]
If life is lost during the end-of-turn after this resolves, it won't be detected. [Shadowmoor FAQ 2008/04/21]
The effect is cumulative. If more than one is in play, the first one will cause extra loss of life. The second one will see that loss in addition to the original loss. And so on. [Shadowmoor FAQ 2008/04/21]
