Question about coupons/extreme couponing?

I was watching the show and i dont understand why its better to buy 70 bottles of ketchup(for example). The impression i got was that they were using one coupon for all 70 bottles and the more they bought the cheaper it made the price. Is that possible? Or are they using a coupon per bottle? If they are only using one coupon did the cashier have to rescan the coupon 70 times?