Indeed, for the past couple days I've been watching, and rewatching, this vid and trying to figure out how a person's legs and feet can move that way. (It gets especially amazing around the 1-minute mark.)
Bottom line: there are times when it would be great to see a video in slow motion. YouTube offers this option, but it's not exactly out in plain sight.
To adjust playback speed if you're using the Chrome browser, click the Settings icon in the lower-right corner of the playback window, then click the Speed selector. Choose 0.25 for quarter-speed playback or 0.5 for half-speed. (Obviously you can also opt for faster playback if you wish.)
Don't see that option? While I found it in Google Chrome, it wasn't visible in either Firefox or Internet Explorer. However, you should still be able to access it in those browsers by enabling the HTML5 version of YouTube's video player (click "Request the HTML5 player"), or by using the site discussed below.
Want more options? Copy the URL of the video you're watching, then head to YoutubeSlow. Paste it into the URL field and then click, "Slow it!" After the video loads, you'll see a Slow Motion button atop various speed selectors. And below that, there are sliders for fine-tuning the playback (including added pauses between each step of the slowed video).
What's the best YouTube vid you've seen that benefits from slow-mo?