New YouTube 5.0 UI (top), old YouTube UI (bottom)
YouTube is rolling out an update for its Android users that will have you easily scrolling through the new navigation menu, flip through videos quicker with the new card user-interface, and multitask while watching videos with the new picture-in-picture option. You just need to swipe the video down (or tap on the check mark in the upper left corner) to minimize the video you’re watching while you browse YouTube and you can close the minimized video quickly by swiping it to the left. It isn’t a system-wide option (meaning it doesn’t work outside of YouTube) but it is a start.
Source: Phandroid