It is pretty much that simple, however the warnings they give aren't as inclusive as they should be. Jumping the PSU like that means it will always be on unless you shut it off using the hard switch on the PSU or pulling the plug, it doesn't use the power switch on the PC. In fact you need to be absolutely sure that you power this auxiliary PSU on before you power on your PC and power it off after you power off your PC. If you don't then you could damage your system by powering on/off devices that aren't meant to be hot-swappable on the board.