Dark arts on power Slash adds even more Enmity than it normally has and doesn't require the combo. That means for quick aggro you could dark arts without it(but that is dumb) or add it on the combo for toms of aggro. Thus counters he fact that dark Knights don't spam the Enmity combo while tanking and let's them focus on soul eater spam. Otger thanks use their Enmity combos more often(pal for debuffs and war for damage)