Downgrading PCI-E x16 to x8?

[pwnyboy]

I figured I'd take this up on the forum, as I saw someone ask a similar question on IRC without a plausible answer.  With a MSI 890FXA-GD70 (an AMD 890FX chipset motherboard), the manual says that if you arrange the PCI-E slot utilization according to their printed recipe, the slots will all become x8 instead of some wacky combination of x16, x4 and x0.  So when the system makes all of the slots uniformly x8, what's actually happening?  Are the additional lanes being physically disabled, i.e. you can't use them?  Or are the slots simply being given the equivalent of x8 throughput with the chipset, even though you can still transact information on all 16 lanes?