Twitter introduced a new way to buy video ads that essentially gives advertisers the option to pay only for completed views.

On Monday, Twitter announced the new method of bidding on ads, which lets advertisers set a goal of getting viewers to watch for at least 6 seconds. If a viewer scrolls away from the promoted video before the 6 seconds are up, the advertiser doesn’t pay.

This makes it technically Twitter’s first ad offering that charges only for completed views, because advertisers can create 6-second ads, an increasingly popular format especially on social platforms.

“It is equivalent to a completed view if you’re running 6-second ads,” says Dan Kang, group product manager at Twitter.

Brands can apply the 6-second minimum to videos that are up to 15 seconds in duration. If videos are longer than 15 seconds, the brand has to rely on a lower viewability standard.

Until now, there were three goals for advertisers to set in Twitter’s ad platform. They could pay for every view, no matter the duration, so long as the video had started. There is also the Media Rating Council industrywide standard of 2 seconds with the video player at least 50 percent on-screen. Then there is Twitter’s previous highest standard of 3 seconds with the video player 100 percent in view.

Advertisers set different standards depending on the types of campaigns, their goals and their budgets. The lower the view standard, the lower the price.

The 6-second views will cost more, Kang says. “The slight trade-off comes when the bid type is a little bit more expensive than others,” Kang says. “But the gains that you get in terms of an uplift in completion rates and an uplift in engagement rates, it far outweighs the incremental costs.”

Viewability has been an ongoing concern among advertisers and platforms in digital media. The issue is when ads should count as being seen by consumers online, and it’s a line that Twitter, Facebook, Google and all publishers have had to define, often with the help of industry groups like Media Rating Council.

The Media Rating Council’s 2-second threshold has long been seen as the bare minimum of what’s acceptable, but the platforms have come up with their own standards, too, such as Twitter’s new 6-second rule. Like Twitter, Facebook has multiple ways to bid on video ads depending on the goals, including the 2-second Media Rating Council standard and completed views.

Twitter’s ad platform is maturing, and video has been one of its fastest-growing segments. In the second quarter, Twitter ad revenue hit $727 million, an increase of 21 percent year over year.

Advertisers can run pre-roll ads, which play before videos from media partners, like a commercial before National Basketball Association highlights. They can also promote their own tweets with videos embedded in them.

Besides giving advertisers a chance to capture more attention with their ads, the move is a way to encourage brands to create shorter videos.

“We also want to introduce some understanding that creative plays a huge part in getting users engaged,” Kang says.