It uses poptop, an open source package that uses a modified pppd and a few kernel modules to emulate a Microsoft VPN server. The beauty of this, and the struggle to make this work, is that the stock VPN client of Microsoft Windows is enough to tunnel in. (retrospect- If we had known Gentoo could just build it for us automatically, we wouldn't have felt so grand when it finally started working by scratch)

If you are interested in making your own VPN server using the same software we did, check out Poptop. http://www.poptop.org/