Out of curiosity, what is that part for?

?lock for bottom seat post clamp?
One could snag lower clamp with leg on back pedaling, and open it.

I really see no purpose for this bike , and I have a Mobiky Genius.

Perhaps that lever locks/unlocks folding of the rear triangle. The hinge and latch are behind the seat tube, with the hinge down by the bottom bracket and the latch at the seat stay. Maybe there's a connecting link between the lever and the latch.

Or maybe it's just the seatpost clamp.

It's to lower the seat post. It's actually very nicely done, and competes well with the Brompton. It's definitely not as small, but it works well. The only issue I see is the plastic parts being fragile. And I see plenty of reasons for this bike, as do some others. The fold, compactness, and portability are quite nice. I just think it requires refinement. And YouTube has TONS of videos showing what 99.99% of these parts are for. Learn to look this stuff up,...it makes it easier to show others what's what.