Every code gets you one dollar off any rental. Most rentals are only one dollar. The exception being their hot releases which are 2.99.
If you have a code go up to the machine and pick out one movie to rent, add promo code and check out by swiping your credit card.
If you have another credit/debit card you can do that all again using the same code with the other cards. Do this as many times as you have credit cards.
So 2 codes and 3 credit cards gets you 6 free rentals if you stick to the $1 rentals and do them individually.