Twitter is tweaking the mobile ads developers buy to promote their apps to make them more attention-getting and fairly priced.

The company will soon let developers append videos to mobile app promotion ads so consumers can get a better idea of the apps in action. The so-called video app cards will play automatically once Twitter officially rolls them out, though advertisers won't be charged for those video views.

Twitter is also refining the way it charges for mobile app promotion ads in the first place. So far, developers have been charged any time a Twitter user clicked on their app install ad, opening up Apple's or Google's mobile app stores, whether or not an app download actually followed. Soon Twitter will give developers the option to only pay if a person actually downloaded their app.

The new option is possible because Twitter's relationships with companies that track app installs have matured, said Richard Alfonsi, Twitter's VP of global online sales. The company's mobile developer toolbox Fabric, which was announced last October, also offers conversion tracking, he said.

Apple's plan to stop letting companies like Twitter see which apps people have installed on their phones or tablets won't have any effect on the company's ability to charge advertisers for actual installs, Mr. Alfonsi added.

While paying only for actual app installs sounds completely sensible, it can also cut an ad's reach because Twitter's technology will try to aim those ads at people most likely to install the app and make Twitter money. As a result, Twitter will offer an option for advertisers to prioritize installs without inhibiting how many people might be shown the ad.