The hindered rotation is due to the hydrogens at the naphthyl moiety. The following shows the rotation calculated on the semi-empirical PM6 level of theory.
We can observe three minima and three transition states corresponding to that rotation, the depiction you have posted is slightly inaccurate as it shows one local minimum and a transition state.
From the three states that should be observable, the global minimum is achiral (1), while the other two are enantiomers (2,3). The latter are connected via a low lying transition state (TS-2-3); probably because of the hydrogen bonding interaction guiding it. The global minimum on the other hand is separated via a very high energy barrier. These transition states (TS-1-2, TS-3-1) are again enantiomeric to each other.
The calculations for these states (on a better level of theory) are still running, therefore I'll post a schematic representation based on the above preliminary scan.
