Mark,

The small reserve tanks on each wing tip just gravity feed into their respective main tank.

The fact that they are just a bit higher due to wing dihedral is what makes that possible.

I often wondered if I opened the reserve crossfeed with a full main what would happen.

The handbook logic was that there needed to be enough room in the main for the fuel to fit into the tank.
Since the fuel vent is right at the wing tip it is logical that fuel could leak out, but I never tried it!  :o

Lou