what in the … ?

I was looking at Apple’s Xserve store page today and noticed the following:

Redundant power supply option

For extra peace of mind, you can add a second hot-swappable power supply and connect each to a separate UPS or power circuit. In case of an outage in one circuit, the second supply takes over without a loss in service.”

Huh? If you have only one power supply, how is it “hot-swappable”?