How do I make the video small and keep watching?

Tap the round button with a down arrow at the top left of the video. The Watch screen closes and the video shrinks into a small player above the tabs. It keeps playing. You can also drag down on the video, or tap the dimmed area above the Watch screen.

Tap the small player, or drag it up, to open the Watch screen again. Slide it down to close it and stop the video.

See the mini player.