The heso is the pyramidal cavity carved in the underside of a floor goban.
Some say that it also improves the sound quality.
axd IMO, I cannot imagine how such a relatively small cavity in such a big mass of wood can contribute significantly in the sound of a stone put on the goban. But I'm note sure.
"6. Navel (blood reservoir) carving
Go to Japan when he is transmitted (the mid-5th century), the board is there to make a box. The Remains of the back hole in the center had been opened for the sound effects. Blood reservoir (No Chi) Name of the samurai in the game of Go was the accomplishment, the spectators came from the mouth (first eight came through clearly), cut off by a samurai sword to the neck of the angry flip board , put the head of the blood collected in the navel, there is a disturbing episode."
6. Navel (blood reservoir) carving
When Go was brought to Japan (in the mid-5th century), the boards were made out of boxes, and a hole was cut into the center of the underside to improve the acoustics. The navel is a remnant of this tradition.
The other name for this indentation, "Blood Reservoir", comes from the story of an accomplished samurai's game. One of the spectators offered an unsolicited opinion as to where one of the players should play next, and the furious samurai beheaded him and placed the spectator's head on the overturned Go board, filling the indentation with blood.
-  http://www.ymimports.com/Store/Tab.aspx?tabid=4#boards : picture of heso