Why are Satella/Envy's unseen hands tied to the Sloth witch factor

Not sure if this has been asked before but I just finished EP 4 and 5 of season 2 part 2 and these hands confuse me. Is it for aura? Can the witches share any other abilities? If anything's answered in the web/light novel of this arc or I just forgot something do tell me. But if it's a major spoiler just ignore this post so I can look forward to an explanation :)