I just found out if you type j when watching a YouTube video, it lapses back about 10-20 seconds, depending on the video, I think.
What are the keyboard shortcuts when using YouTube?
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It's at the bottom of the Screen reader support page:
Keyboard shortcut Function ------------------------------------ O To jump to the beginning of the flash player Spacebar Play/Pause when the seek bar is selected. Activate a button if a button has focus Play/Pause Media Key on keyboards Play / Pause Stop Media Key on keyboards Stop Next Track Media Key on keyboards Moves to the next track in a play/list Left/Right arrow on the seek bar Seek backward/forward 5 seconds Ctrl+left/ctrl+right arrow on the seek bar Seek backward/forward 10 seconds Home/End on the seek bar Seek to the beginning/last seconds of the video Up/Down arrow on the seek bar Increase/Decrease volume 5% Numbers 1 to 9 (not on the numeric pad) on the seek bar Seek to the 10% to 90% of the video Number 0 (not on the numeric pad) on the seek bar Seek to the beginning of the video
Indeed it's very hard to find. They ought to make a dedicated page for this.
Note: These shortcuts will only work when the video player is in focus. To move the focus try pressing the spacebar to pause and then play the video.