support for "bow" window
With the "cable system", there are metal rods in the frame between windows that go the full height of the window and are tied into the frame. You connect a cable to an eye at the top of the window and run it back at an angle to the wall the window is installed in, above the window. The other end of the cable is fastened to a bolt that is securely fastened to the framing of the wall (this may be a lag bolt or a through bolt, depending on how the wall is framed). 

I don't think these windows have this feature however. If you go to Support > Installation Instructions for the windows you posted, you'll see this quote (emphasis mine): 


  IMPORTANT: While Integrity Windows and Doors does not
  control the style of platform support to be constructed, all bow and
  bay applications must have support located beneath the unit that
  follows the angle or radius projection of the unit. The support
  platform MUST extend directly under the sill of the unit to provide
  continuous support. AN INTEGRITY BOW/BAY IS NOT IN ANY
  APPLICATION ABLE TO SUPPORT ITSELF.


So if these are the windows you're getting, it looks like you're stuck with the support under the bottom.