Now I have tried the new curse on the training dummy I can answer at least this question I posed. Yes “falls off” and “times out” appear to mean the same thing. Taking the Acute Affliction node, I was able to trigger Rip Blood immediately by casting Soul Feast once (with the “up to 6 extra hits” node). The trigger occurred whenever the cast of Soul Feast applied at least 4 hits. However the trigger effect is bugged as follows:
a) Rip Blood never once triggered early with Wandering Spirits with Spectral Putrescence, hitting the dummy, despite the tooltip for this node making it clear that the spirits “hit” with this ranged attack (it applies poison “on hit” after all, and the spirits are not Minions). Acute Affliction tooltip does not state that the 4 hits to trigger Rip Blood need to be applied by me, so this should work. I also assume in multiplayer, the hits dealt by other players should trigger it but obviously I cannot test that.
b) If your first cast of Soul Feast does less than 4 hits (hence no trigger of Rip Blood), casting Soul Feast a second time will NOT make the Rip Blood trigger no matter how many hits the 2nd cast does.
c) If you do: Curse -> Soul Feast -> Soul Feast and the sum total of hits from the two Soul Feast is less than 4, then the Rip Blood will never trigger, even when the curse times out after 8 seconds.