I'm assuming you get checkpoints after every loot drop. Of course then there's the question of "what if I disconnect after killing Rixis but before picking up loot" or "what if I get a checkpoint and want to keep re-rolling for loot?". I guess you'd get a checkpoint after killing an enemy that drops loot, and another right after you pick it up and find out what that loot is, to prevent exploiting the system to reroll for loot you want. Enemies would always drop loot of a certain tier, so that'd make rerolling for, say, a gold loot orb instead of a purple one a moot issue. And it would just logically follow that checkpoints outside of Public Events, that occur before/after you enter/exit those areas, are likely where you restart from if everyone dies in those.