They did it by providing a huge docking connector which exports the entire internal peripheral bus (PCI or PCIe) to the docking station, plus additional connections for power-input, ethernet, and external monitors from the internal GPU. The docking station can then add additional PCI/PCIe chips onto the bus to support extra ports (eg. serial/parallel, more USB root ports, whatever).

Thunderbolt does mostly the same thing using far fewer pins, and tries to standardize it.

EDIT: My main machine here is a Dell Precision M6300. It has an old school docking station (fantastic!), but also has an ExpressCard slot --> essentially a hot-plugable PCIe 1X expansion slot. I use that slot to add eSATA ports, USB-3.0 ports, a real bus Serial Port, and other nefarious add-ons as needed.

ThunderBolt is the new version of that slot, on steroids!

Cheers