They pretend (or pretended) to have coupons that you needed to click in order to reveal. Often upon clicking, the coupons were invalid. Perhaps some were never real. But upon clicking, a pop-under would launch which would drop their cookie.

So a user who was already committed to buying, and in the process of doing so, who simply checked for a coupon while going through the checkout process, would end up generating an affiliate commission for RetailMeNot even though RMN provided no value to either the customer or the merchant.

This is basically a scam that pretty much all coupon sites pull, which is why many companies ban coupon sites from joining their affiliate programs.

They don't actually refer anyone (usually you don't look for coupons before you're at the checkout window), but in order to view a coupon you need to click a link that gives you the RetailMeNot referrer cookie, giving RetailMeNot a cut for doing basically nothing.

The scam back in the day* was that they would bid on their retailer's own keywords. Their ads would send that traffic through their own affiliate links and get paid on any revenue generated. The ads looked legit and appeared to point to the retailers so most folks never even knew. So they cannibalized their retailer's own paid traffic and made it more expensive to bid on their own keywords.

*Back in the day was nearly ten years ago. Things may have changed.

I see other folks have already explained what I would call the "pop-under" approach where a new window is opened from RMN's site to the retailer in question. This is nefarious but in a slightly different way than the above.

RetailMeNot is a weird name isn't it? "don't sell stuff (to me)" for a site for selling stuff (to me)!?

Huh, totally forgot about that! This was at least partially due to the fact that "WhaleShark Media" had become synonymous with large-scale fraud in some affiliate programs around this time.

It actually started as RetailMeNot, a website run by two Australians. Whale shark media bought it, and then renamed their company to RetailMeNot. The website has always been RetailMeNot.com.