Digital BW, The Print

With cameras like the Nikon 8400 used in BW mode, Nikon says that the 
file is the size of a color file and that the "extra" data is used 
for "a higher level of detail."  What does this mean?  

I recently saw a full 13 x 19 print from the 8400 shot in BW with JPEG 
extra (2:1) compression and the image was literally stunning--flawless 
to my eye.  The detail and smooth tonal scale looked like something 
shot on Pan-X.  (The image was a difficult one too, with some black 
objects in the interior but near a window and white lines on the street 
outside in the sunlight.) Unfortunately the BW mode cannot be used for 
RAW shooting, though I imagine the camera is too slow to really use RAW.

So what's with the BW mode?