That's it. Just show the cashier a recent receipt from any gas station and you can get yourself two free tacos. The deal runs all day and you don't have to buy anything else. However, the receipt must be original, no copies allowed. Also, only one order of tacos per gas receipt.
If you live near a Jack in the Box, keep your gas receipts and have a free snack on Thursday. I'd probably do that if I lived anywhere near a Jack in the Box, but I don't think there are any of them on the east coast.