The tricky strategy of all-you-can-eat.

For a while this summer, it looked like the moderates might win out in the tug of war for the minds and stomachs of American diners. The culinary arms race of ever-larger portions had started to taper off, supersize became a dirty word, and commodity prices—including those of many basic ingredients—were skyrocketing.

But then consumers began slamming the brakes on spending. Restaurants, by nature a discretionary splurge, are feeling the pinch. Third-quarter same-store sales were down 2.5 percent at full-service restaurants, according to market research firm Technomic.

To compete in a newly cutthroat landscape, restaurant-management companies are turning to that relic of retirement-community buffets and 1980s-era Las Vegas: the all-you-can-eat plan. In recent months, several nationwide casual-dining brands included all-you-can-eat pricing options on their menus, and there's evidence that local chains and mom-and-pop eateries are also running promos encouraging customers to belly up to the trough. Pancake purveyor IHOP offered all-you-can-eat buttermilk pancakes for five bucks, and seafood chain Red Lobster rolled out an all-you-can-eat shrimp deal.

The tactic is a bid to keep newly budget-minded customers from defecting to the drive-through. Such trading down is a perennial thorn in the side of casual sit-down restaurants during downturns, although they try to lure fine-dining customers to trade down themselves. The marketing director for Cici's, a Coppell, Texas-based pizza chain that operates entirely on an all-you-can-eat buffet model, says its customer feedback indicates that a growing number of diners are trading down from pricier eateries.

What's the appeal of all-you-can-eat for the restaurants? Despite the potential for abuse (there are entire communities of blogs dedicated to the task of socking a restaurant for as much as possible over the course of one all-you-can-eat visit), it does have a track record for getting people in the door. Better yet, it's inevitable that some of these diners will change their mind before ordering and opt for higher-margin a la carte items. Even if one member of a party sets out to gorge on the deal, the rest of the table will most likely order a la carte, so if the restaurant does lose money on individual big eaters, their companions' orders as well as ancillary items like drinks—especially alcoholic ones—and desserts make up the shortfall.

There are other tricks of the all-you-can-eat trade. The stuff-yourself-silly item is usually a cheap, water- or starch-based commodity like soup or pancakes. For instance, Orlando-based Darden Restaurants' Olive Garden brand ran an "endless pasta bowl" promotion this fall, and it offers an ongoing all-you-can-eat lunch option that includes soup, salad, and breadsticks. It's worth noting that Olive Garden out-earned most of its casual-dining peers in the third quarter; its same-store sales rose by 2.4 percent.