The ability description reads:
You can 'purchase' items from the store for as cheap as free. Don't let Brax catch you at it though, or he'll send his Dread Collectors after you.
The wiki agrees:
Five-Finger Discount allows you to steal an item from Brax's shop by destroying the pedestal below it. Brax might notice you and turn hostile, which more likely than not leads to Fun.
OK... so how does this skill actually work? Is it just a random dice roll to see if Brax notices me or not? Or is there something specific I can do to assure success? What happens if I stand in a location which Brax cannot see (is there such a location in shops)?