It keeps track of your spending whether you lose or win. Just show your ID and choose a password, then enter your rewards card in every machine you gamble on.
This only applies if you have a Foxwoods rewards card, which is available for free at the Rewards Desk inside the casino. Both casinos have lousy odds on their slot machines so since you're bound to lose your money, you might as well lose it at Foxwoods where at least you'll get something for your money spent.
The distance between Foxwoods and Mohegan Sun is about 15-20 minutes by car. Yet at Foxwoods, for about the same amount of money spent, we were given several complimentary offers. We found that we could gamble considerably at Mohegan Sun and not get comped anything.