It is supposed to inflict an extra damage if target's HP is low, not exactly increase the damage of your spell.
Greetings
I just tested that, and nothing happens. I hit a mob below 15% and after that just used white-hits (so I don't kill it with 1 shot). The mob was below 15% for over 10sec and nothing happened.
White hits aren't included to this artefact's passive, but this may be changed in next patch.
Greetings