[GRASS-user] meaning of -n flag in v.surf.idw

[Luigi Ponti]

Dear list,

I am having a hard time figuring out what does the -n flag do in v.surf.idw.
 From help, I read:
-n
    Don't index points by raster cell (slower but uses less memory and 
includes points from outside region in the interpolation)

Compared to a raster produced with -n enabled, without the -n flag the 
range of data changes (narrows), and strange band-like artifacts come 
out in the display (i.e., surface is not very smooth). There is a mask 
set, and all the points are included in the region and within the mask - 
the only thing I change is the -n flag.

Is anybody able to clarify what is the algorithmal change introduced by 
the -n flag?

Thank you in advance,

Luigi