What does `pcie_aspm` do?

So, the first hit on Google for "pcie_aspm" is:

[patch] pci: document the pcie_aspm kernel parameter

and that documentation is in the current kernel Documentation/kernel-parameters.txt, where it says:

    pcie_aspm=      [PCIE] Forcibly enable or disable PCIe Active State Power
                    Management.
            off     Disable ASPM.
            force   Enable ASPM even on devices that claim not to support it.
                    WARNING: Forcing ASPM on may cause system lockups.

And in turn, google for "pcie active state power management" leads to PCI Express* Architecture Power Management document, which explans that ASPM does pretty much what the name implies.