The switch comes loose with every Pine64 board, because some backers wanted it and others didn't. You can solder it on the "Power" or "Reset" area found on the right side of the image here:
http://wiki.pine64.org/images/7/7d/Pine6...nector.png
It's totally optional, you don't need it to use your board, but some like it as a convenience.
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I didn't notice one in my box when I received it, so I just took a look on the floor around my desk where I opened the package and I found the switch under my desk. Must have fell out when I opened it.
Currently playing around with Arduinos, CHIPs, Orange Pis, PineA64s, Raspberry Pis, etc.