With attacks of opportunity, the PHB is pretty clear that your reaction occurs as an interruption to your opponent's move, just before they move beyond your reach, then they resume their turn. Fair enough. 

However, there are some abilities that happen during an opponent's attack - for example, the Shield spell, and a Tempest cleric's Wrath of the Storm ability. These both say they happen when you're "hit" by an attack - but the Shield spell raises your AC, so I assume that occurs before rolling damage, and makes it possible for you to be retroactively _not_ hit by the attack. Is the same true for Wrath - that your reaction might KO your opponent, and you wouldn't have to worry about taking damage? 

(I was planning to include Hellish Rebuke in this question but I see it specifically refers to "being damaged", so I assume that means it happens on your opponent's turn but definitely after getting hit, meaning you can only cast it if you're still conscious.)