YT Zoom works automatically. Just click "Add to Chrome," then visit any Youtube video and click magnifier button to zoom your video.
Additionally, you can move a zoomed video with these buttons:
Ctrl + ⬅ - move viewpoint left
Ctrl + ➡ - move viewpoint right
Ctrl + ⬆ - move viewpoint up
Ctrl + ⬇ - move viewpoint down
Ctrl + Backspace - reset viewpoint to center
谷歌浏览器、其他Chromium内核的浏览器