According to several reports from Twitter, it seems that YouTube is testing the feature out with a small group of users. It is unclear how they are selecting these users, but given that it is a small group, it’s safe to assume that they are testing it and probably won’t be rolling it out to the masses anytime soon.
Some users who have received the feature are also claiming that the feature is a bit limited where it apparently only works with live streams as opposed on-demand videos, but like we said, given that this is probably a test, it makes sense that YouTube is probably limiting the scope to see how well it works out.
It is a rather useful feature to have and would be great if iPhone users could continue watching YouTube videos while using other apps on their phones, so hopefully YouTube will roll this feature out soon.