Q:

How does Five-Finger Discount work?

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)?

A:

Five-finger discount has been changed, and as of version 1.09, automatically gives you a free item when you enter a store for the first time. I guess the previous version was too risky.

It doesn't actually take anything from the shop - all the pedestals still have items on them even after you steal it. It just gives you a random item, which, I assume, is on the same "quality range" as other items in the shop.
There are no repercussions, Brax doesn't get upset and you can sell the item back to him right away.
It you visit a store, then learn the skill, and then come back to visit it, you don't get to steal something. It has to be an undiscovered shop.
If you have no free room in your inventory and you visit a shop, you will not get an item - it will not appear on the floor.

A:

As far as I can tell, it's a random dice roll.  Failing the dice roll will count as shoplifting, which causes Brax (and all the other Brax's on the level) to become hostile and summon Dread Collectors after you any time you're on that floor.  Killing all the Brax will stop the flow of Dread Collectors, although you'll still have to deal with any stragglers.  
On other floors, the Brax clones won't be hostile, as long as you don't steal from them.
While there are (occasionally) squares in shops that Brax can't see, this doesn't stop him from noticing you if you're trying to shoplift, or if you have 5 Finger Discount and you fail the roll.  He's got eyes in the back of his head, I guess.  (or maybe some really good security cameras?)

