Here's a quick guide of how it works for those that can't view the video at this exact moment:
- Completely unplug and power down your PSU.
- Locate Clippy. Stick one end of Clippy into the slot with the green wire, one into a slot with a black wire.
- Set the cable aside. Plug the PSU in and flip the switch. It should spin up.
- If it doesn't spin up, it probably needs to be put under some kind of load -- connect a fan or two (or a HDD) and see if it works now.
The beauty of this is that you don't need to hook it into a computer. It also helps isolate issues when troubleshooting - it doesn't definitively test a PSU as good or bad (nor is it meant to), but it does help figure out if a power failure or startup problem is a result of the motherboard, PSU, or something else.