it rotates through aspect ratios. normal, grey, stretch, partial zoom and zoom. i use normal and Partial Zoom. i think it has most value on a 16:9 screen
Guess I'm unclear on what you mean by partial zoom, on the 921 you get gray bars, normal, stretch, or zoom on SD channels and normal, stretch or zoom on HD channels.
all i can tell you is i get a full screen on 4:3 images, i lose a little off the top and bottom and it doesnt appear to be stretched. There is a 942 review that states it was added back into the 942, as per the 6000.
The 6000 partial zoom mode is a compromise between "Stretch" and "zoom". It crops a bit from top and bottom and then stretches the remaining picture uniformly.