So I've needed to test the fuel cell and figure out what happens if I try to overflow it. The recipe is simple: fill the main tank, fill the aux tank, open the transfer valve, leave the bike on the sidestand.

Thirty minutes later, there was a gallon of fuel soaking into the garage floor (see below). Apparently, the overflow is out of the top of the main tank. So all the fuel came out and drooled across the pretty paint of the tank. Oh well, at least I know not to leave the valve open now (on some bikes, you can leave the transfer valve open, and the venting will keep it from overflowing).