YouTube has been reluctant to enable Picture-in-Picture mode on the iPhone, after the feature became available in iOS4, enabling and disabling the feature multiple times.
Now the company is giving it another go, and are inviting YouTube Premium subscribers to try out PiP mode as a new experimental feature.
To access PiP mode, visit YouTube.com/new while logged in to their Google Account, scroll to “Picture-in-picture on iOS” experiment and select “Try it out” to activate the option.
Following this, if you return to the home screen while playing a video, it will continue playing in a mini window. If you lock your phone the video will stop, but you can resume playback using the lock screen controls.
The experiment will remain live until the 31st of October, after which your mini window will presumably disappear until the next experiment.