how does dynamic contrast work on Plasmas?

[maygit]

I know lcd's adjust the backlighting in their sets to produce higher contrast levels but how does this work in plasmas? I see the panasonic plasma th42px80u has a 15,000:1 contrast but 1,000,000:1 dynamic. Suprisingly the black levels(to me) don't really look near what I had imagined a 1mil:1 contrast would be. Infact, i think the sony xbr4's look way darker with a mere 18,000:1. So how do u feel about dynamic contrast in plasmas and most importantly, how does it work?