Q:
How does Mug work?
A:
its a passive addition to your steal, without it you just port to them. With mug traited you hit for dmg and gain health as well when you steal. Watch your damage log youll see it.
Yeah basically what splatter said. Your steal does not usually do damage, it just usually steps you to the enemy and steals a skill. However with mug steal does damage, and heals you.