Connect my amp out of phase?

Yes, that is all. Just in case you don't fully understand, they mean only at one end. You can either reverse the speaker wires at the amplifier end, or at the speaker end (same result).

Why is this? What is it about this preamp that makes this work?

There are a number of threads on reversing polarity. There is something called Woods Effect that some hear when polarity is reversed. Many people don't hear it. Whether a component (this is not unique to pre-amps -- amps, DACs, etc can reverse polarity, and things can be recorded with reversed polarity) reverses polarity just has to do with how the circuitry is implemented.