SPI question on BF526

I'm trying to get the bf526 to talk to the AD7908 over SPI.  It looks like I need CPOL = 1 and CPHA = 0, but when I choose this setting my select line stops going low when I'm trying to do a write.  The 526 manual says that when CPHA = 0, the SPI slave select line is automatically controlled by hardware.  Why?  What line is 'the SPI hardware' supposedly driving low?  What if I need these CPOL and CPHA settings but also need to be able to control which slave select line I want to pull low via the SPI flags?  Can someone explain?