The ones I am familiar with is where the flier has a scratch off w/ a number and the claim that you won a $20,000 prize or whatever...
...only you have to go to the dealership to view the number on display. If you're number matches, you win the $20,000.
Legally they have to mail a winning ticket in accordance w/ whatever odds they have printed, so the gimmick is the direct mailing company simply mails the known winning flier to a bunk address
I've also seen the free ipod, cruise or vacation package ones; EVERYBODY wins the vacation package; which entails some stupid fees and a timeshare seminar
Although once I handed out hundreds of $5 Home Depot Gift cards from a car sale flier promotion...I have no idea what THAT campaign costs