Here's how the Stop & Shop thing works:

At the door there are a rack of hand scanners. You scan your store card and it activates a scanner (by lighting up around it) that you take and put in the holder on your cart (made just for this).

You take a bag(s) and put these in the cart also.

As you shop, you scan each item, then place it in the bag. If you scan an item and decide not to buy it (more expensive than you thought, for instance), you push the remove button on the scanner and then scan again and it takes it off your bill.

When you get to the checkout (self or regular - I've only used the self checkouts), you scan a barcode marked end of order, then scan your card again. You then place the scanner in a rack at the checkout (again made for that purpose), it brings up on screen whatever you have bought, you (or cashier) then scan any coupons, choose how to pay and complete the transaction.

While you could put items in the bag, I believe I read somewhere that every so often they may pick a customer (at random?) where they will re-check your bags, which might make people less likely to add "extras". I've used it a few times and it hasn't asked for that (yet).

This is the only store I've seen something like this at, and only one location near here (so far) - I'm guessing they might limit stores where they place these based on past records of thefts and such?

As noted in another post, this store rarely has a clerk at the self checkouts anyhow (I have seen some S&S that do, so again maybe based on patterns of losses?).

The scanner is also handy, as you can use it like a price checker if needed, rather than having to run all over the store looking for one.