I believe it finds the coordinates of the two points and uses Google Maps to choose the fastest "driving" route. If you're not aware already, you can do driving directions on Google Maps by putting in two sets of coordinates. For example: http://maps.google.com/maps?f=q&hl=e...UTF8&z=11&om=1

Another nifty map tool is http://mapper.acme.com. Although it won't measure custom cycling routes or anything, it's neat to fool around with. Plus it has USGS topo maps built right in, and NEXRAD weather radar. See the elevation and weather on your next ride!