https://www.gamespot.com/articles/big-nintendo-switch-update-released-adds-cool-new-/1100-6454184/

A big new update for the Nintendo Switch is out now. Update 4.0.0 adds a number of notable and welcome features, perhaps the biggest of which are video capture support and the ability to transfer data to a new system. It's a big update. [Update: It turns out there's more to this update than expected, as it introduces wireless headphone support to Switch. Additionally, a new Mario Kart 8 Deluxe update is out, adding support for the new video capture feature.]

With the update applied, you can hold down the Capture button and this will record the previous 30 seconds, saving to your album. You can then edit your video and share it to Facebook and Twitter. Supported games include The Legend of Zelda: Breath of the Wild, Mario Kart 8 Deluxe, Arms, and Splatoon 2. This feature is not working yet for Mario Kart 8. An message in the Switch's news channel says the feature is supported at Mario Kart 8 Deluxe version 1.3.0, but the current version is 1.2.1. So it sounds like the feature will be unlocked later in the racing game. Nintendo also says "many more games" will add support for video capture in the future.