It's the ESC plate. Something has to swing away, to remove the battery.

And any chassis that has two horizontal plates (chassis plate/top deck) can adjust flex by loosening screws that attach the top deck to the uprights it's screwed to. removal of certain parts can also do the same thing, but must be done carefully, so overall strength is not compromised.
A chassis that uses vertical plates is harder to adjust, if it can be at all, because the plates being vertical makes them act as trusses. The RSector doesn't allow for flex, because there are two thick vertical plates that have no play, even if they're loosened (believe me, I checked!).