My bad, I thought iwas a clutch-plate diff, not a helical diff like you have clarified. Essentially it uses friction once again to jam the helical gears into the case.
Clutch plate diffs use spider gears just like open diffs do. They just have clutches attached the axle gears. There really aren't any limited slip diffs that do not use some sort of gearing.
A helical gear diff generates friction, as was said, by generating thrust forces which push the gears into the walls of the case. They don't actually move out and push into the case, as they have no where to go. But they are loaded up just the same and push against the case (they just were already in contact with it).
BTW, wheel hop will kill a helical gear diff too, it just might take more of it to do it. The failure mode is different, but it can happen...
