I don't really notice my form is much different. I guess if you sit all the way down rather than just a touch, you do can keep your back a bit straighter, but you'll probably lean forward a bit as you stand again.
The first time I did the sitting down version, I didn't know that I had to lighten the bar a tad from what I usually squat. Soooooo...I loaded it up nice and heavy, squatted and sat down, and there I stayed! Much to the amusement of the guy who came and rescued me
Mel