Yes you are right, just tested it.
I thought it triggered from poison damage, but it seems like it triggers from Rupture ticks as long as a poison debuff is active.
Thanks, that's useful to know.
Edit: It works even if you only having Crippling / Leeching poison applied too.
Edit 2: I also tested the damage on an 85 Dummy, Wound poison hits for approx 5k
Deadly poison hits for 6k, but ticks for 10k