As I was perusing Slate’s homepage Monday, an ad caught my eye. Granted, I’d have to have had my eyes closed to miss it: It was the kind that takes over your screen momentarily, with a high-production-value animation enticing you to check out the advertiser’s site, in this case Auto Trader. I found it alluring enough that I opened up a new browser to watch it one more time. When a teaser for a story by Joel Dreyfuss (“Michael Steele has a point”) appeared at the top of the page, animated cars burst onto the screen, smashing the front-page design. When the ad ended, the front page reassembled itself in a Universal Studios backlot-tour style. Neat! (Or, annoying! Depending on your taste.)

I clicked around the Slate homepage looking for Dreyfuss’ story to see if I could trigger it again, but no luck — in either finding the story or triggering the ad. Josh grabbed a video of it for me, and, as you can see above, the ad launches just after the mystery Steele story takes the top slot. In the real, non-ad world, that story is not actually featured on Slate’s front page: It’s a month-old story published on Slate’s sister site The Root, whose stories are frequently cross-promoted on Slate.

In an email exchange with Matt Turck, VP of advertising sales for Slate Group, which saw a 52-percent ad revenue gain in the first quarter of this year, he explained that Auto Trader chose the Steele story and took the screenshot of it to use as part of the ad. So when you see what looks like a placeholder for the Steele story, you’re actually already watching Auto Trader’s advertisement. It creates a natural flow for the launch of the ad, but with downside of confusing any readers who’d actually like to check out the story.

(That didn’t stop us from having an age-old problem: After watching the ad, we couldn’t remember what it was for. A car company? Some sort of car-related service? I guessed Carfax; Josh guessed Ford.)

Turck said Slate tracks how well campaigns like this one work at getting clicks, but that information is “proprietary to advertisers.” He did say that, generally, ads that break out of the traditional banner box like this work better than traditional ones. “Most successful ad campaigns take advantage of unique and creative executions.”

But, of course, one person’s “unique and creative” execution is another person’s “typical and irritating,” forcing Slate also to consider their audience’s, and balance intrusive ads against fleeing users. I asked if Slate has a set of criteria in place to help strike that balance. “There’s no specific set of criteria, but we run all advertising by editorial first conceptually, and then again just prior to final execution,” he said in the email. “Our editors will always err to the intelligence of our reader base. Fortunately for Slate, our readers have grown up on the Web and therefore are often interested in and embrace interesting ad executions.” (Slate posts its reader demographics and ad services it provides here.) Turck also noted that the intrusive Auto Trader ad only loads once in a 24-hour period for a user, barring a browser switch.