This is an operation known as "crowning the soundboard".   Before a new soundboard is installed in a piano, it is first given a curvature.  (Approximately that of a 50 to 60 foot radius circle). Here the board can be seen in an upside down position, with the ribs being glued to the underside of the board which is now face up in the photo.  Underneath the board are curved forms called "cauls".  The long poles pressing down on the ribs are called "go-bars" and are made of hickory.  They exert a downward pressure on the rib and soundboard of about 160 lbs for each go-bar.  On some boards you can actually use close to a hundred of these go-bars, which requires the use of a special clamping table known as a go-bar deck.  The ceiling of this table (not visible in photo), against which the go-bars push, is designed to withstand an upward pressure of over 40,000 lbs or more.  Otherwise, it would literally raise the roof every time a soundboard was crowned.