How exactly does dual +12 volt rails work?

I'm looking around for a new psu and I know which brands to shop for.what I want to know is how do the dual 12volt rails work?does the 4 pin motherboard connector have it's own dedicated rail?does it switch to the other rail when ones loaded?or does it split the load evenly?or does it depend on what's plugged in where?Is there really any advantage over a single rail?and finally which is better 30amps on one rail or split into 2?

Dual 12v rails mean that there is a seperate rail powering the mobo from that powering everything else, so the power is "cleaner." It depends on your system if this would be useful, but it is a good idea.

Thanks that's how I thought it worked but wasn't sure.

12v 1 powers the mobo, 12v 2 powers every other 12v item.