The "dh=true" parameter is telling NNEDI2 to double the height of field0, which results in an overall increase of 1.5x the original frame height. So in actual fact the image does not get up-scaled until this point. ;) Then Spline16 scales up the horizontal slightly IIRC and scales down the vertical.
G-Forces Stabilisation Script said:
########## anti-alias
NNEDI2(dh=true, field=0)
NNEDI2(dh=false,field=1)
Like I said before I found when playing around the other day that the above anti-aliasing section needs to come before any up-scaling commands otherwise it doesn't work properly.