YouTube's Android client is in the process of having a new version of a somewhat old feature roll out on the server side, and that is the ability to double-tap to the left or the right of the main controls to fast-forward or rewind a YouTube video by 10 seconds. The capability has been built into the app for a while, for those willing to toggle an accessibility setting for the app, but those who didn't know about the setting were out in the cold, and it simply added fast forward and rewind buttons to the controls, rather than allowing users access via a double tap.
The new feature is rolling out on the server side, which means that you won't need to update your YouTube client in order to see the update, though it may require a reasonably recent version. As Android's core services and APIs advance, older versions lose compatibility with some apps and newer versions of older apps. Google has not stated what the cutoff is for this particular server side update, but those who are a couple versions back from the latest and greatest may not want to hold their breath on this update. According to Android's latest OS distribution charts, the percentage of users in that boat is incredibly small, so the likelihood of Google putting in the work to implement this change for them is somewhat low.
The feature will be rolling out gradually to users over the next few weeks, and you won't notice an update or any download traffic, since the update is server-side. At this point, there is no word on whether those who have enabled the accessibility setting will still have permanent fast forward and rewind controls present when they use the app. Rolling out the functionality to all fixes a key pain point of using YouTube on a mobile device, being the inability to seek out content you want in a given video and jump between small sections. Those waiting for Google to add actual speed controls into the mobile version of YouTube, however, are still out in the cold.