I'm not even going to ask.
"finish planing the belly" = finish surface planing (using a hand-plane to remove materially incrementally to achieve a smooth surface of desired dimensions and parallelism) the section of bamboo which shall form the belly (front, or the the towards target) of the japanese asymmetric longbow ("yumi") I have been working on for several months ;)
for further quick reference on this type of bow, look here:
http://www.kyudo.com/kyudo-e5.html