Turning the first (quantize) soft knob allows the loop to include more or less of the beat's time steps (AFAIK).
Sorry, I just tested it. Someone correct me if I'm wrong -- I don't really use LoopScreen:
1. Enable LoopScreen by entering 16 Time Steps and holding shift and turning soft knob 2,
2. With LoopScreen enabled, turning the first soft knob (quantize) controls the time step granularity amount allowed to loop in the beat. E.g. turning the first soft knob all the way back displays less time step intervals, hence a longer loop. Turning the soft knob forward increases the granularity of the intervals, hence a tighter loop.
3. With LoopScreen enabled, turning the second soft knob will sort of "scrub" or navigate the loop to the time step columns denoted by the X.X.X column values.
4. Messing with #2 in combination with #3 will determine where the in the beat the loop will occur and for how long/tight the loop will be.
5. The above functionality seems to not work consistently when using non-4/4 time signatures, resulting in silence when some combinations of the #2 and #3 are used.
That's my understanding, I could be wrong, like I said I don't really use it.