I was watching a movie called LEGEND by Tom Hardy. It's around 29:07 in the movie when a man was walking into an office and a female voice-over said
"Ruling London involved intelligence and intimidation.The trick was to cultivate a quiet certainty that should it be needed, violence was on offer and would be happily applied"
Why use "should it be needed" but not "it should be needed" here? What's the meaning of "quiet certainty"? I've done googling and couldn't find any help. Can someone please tell me the exact meaning of this movie line?