X is making the new vertical video ads, shown in its specific video feed, activated by tapping through on a particular video clip in-stream, more accessible.
That's not how you would think most people are watching video in the app, since the vast majority of X's use is actually in the main feed. Apparently that assumption was wrong though, since X claims 100 million users now interact with vertical video inside the app each day.

X says:

"X is becoming a video-first experience, with video now part of more than four out of five user sessions, and video views growing at an average rate of 35% year over year. We are expanding Vertical Video Ads to all advertisers globally, so anyone can have a full-screen, sound-on advertising experience on X's fastest-growing video surface."

Now, exactly how X can call itself a "video-first" platform when it doesn't open to a video feed is another question. But maybe that's coming too.

X says that, based on testing, Vertical Video Ads are the most engaging ad format that it's ever released, though whether that incorporates the history of X and Twitter, or just X, is unclear.

"We have seen X users are 7X more likely to tap into an ad in a vertical video versus a Home Timeline version of the same ad, and early adopters averaged -14% lower CPMs against the same version of ads in the Home Timeline.".
Theoretically, it should add more value, based on video consumption trends. Insights from Twitter showed that, previously, tweets with video generated 10x more engagement than non video posts, and X claims that, on any given day, vertical video now accounts for more than 20% of total platform usage.

That feels like a lot, but if that's correct, then there should be a level of opportunity in this new placement.

Now, will X actually use this trend towards even more full-screen video and make it a key engagement play by opening to video feeds or as an alternative UI option sometime down the line?  ONLY time will tell.

That being said, based on these stats, its new vertical video ad option might be worth testing.

X indicates that all U.S.-based advertisers will be able to launch vertical video ad campaigns starting today.