Crowning one rib at a time -- Spreeman

[Richard Brekne]

Was reading about the Ravencroft instrument and have been curious about 
many things since first hearing about it. On the website it says 
somewhere that the soundboard is crowned "one rib at a time".  I wonder 
if you could be so kind as to expound on that a bit.  Exactly how do you 
crown a board one rib at a time ?

Thanks if you can find the time to explain
Cheers
RicB