It not supposed to require soldering. It is intended to just slide it over the pins of the header, which allows another extension board to be put on top of it.
However, in my experience during the past 8 months the contacts of the board to the pins of the pi is not very reliable. Quite often I found the pi not to be able to detect a hardware clock. Fiddling a bit with board most often resolves that, but that's not what you want and over time it gets worse. Since soldering would make it impossible to remove the board from it's case I just soldered a female header to the clock.
According to their website some of the holes are offset to create a friction fit. In my opinion, this will become unreliable over time especially if you keep removing the board for whatever reason. I agree with the others, either solder directly to the pi which would make it difficult to remove or solder a socket header to the rtc which then stops you adding other things to the pi connector.
