Just did a small bit of testing with it last night, was lagging like hell so only managed some results.
I was using Raptor pet with S.Scythe, V.Violet and R. Ruins atma, which puts Raptor to about ~4.8k hp?
Anywho, it seems affected by pet max HP, damage didn't alter between raptor at full HP and raptor being injured.
It seems to be able to proc on WS but using Rampage I can't 100%, average Rampage was 1.4-1.6 with the odd 2-2.5k on Crabs (DC-T, was trio)
My axes aren't the same base damage, Gui +1 with a Gui nq, both pet: eva axes.
Can provide SSs of the gear I was using if needed/etc D:
Main = mainhand Off = offhand DA = Double attack, etc.