The technology will kick in whenever three or more cores are idle and power consumption is below the processor's rated TDP. When that happens the idle cores are set in a boost-eligible "P-state" and their frequency is reduced to 800MHz, while the voltage to the entire chip is increased, and the other three cores are clocked up by as much as 500MHz. AMD says this will allow Phenom II X6 processors to fully utilize available TDP budget to maximize performance.
Keep in mind these chips will remain within the same power envelope as current Phenom II X4 parts, despite having two extra cores and still being manufactured on a 45nm process. AMD's new Phenom II X6 and forthcoming quad core CPUs based on the new Thuban design won't require new motherboards, as they'll be compatible with existing AM3 and AM2+ sockets via a BIOS update, though it isn't clear if Turbo Core will be limited to newer AM3 boards.