We have performed detailed temperature-dependent study of optical f-f transitions of the Yb3+ ions in h-YbMnO3 by means of Fourier-transform spectroscopy. The splitting of the ground Kramers doublet as a function of temperature, 0(T), was determined for the Yb3+ ion at 4b site. The 0(T) function follows the dynamics of the manganese magnetic moment below TN = 87 K, indicating that the ytterbium subsystem is magnetized by the magnetic field generated by an ordered manganese subsystem, which is consistent with the results of previous studies. Excitation of the upper component of the split ground doublet plays a significant role in the low-temperature dynamics of the h-YbMnO3 crystal. Using the 0(T) function, we calculated the temperature behavior of the Yb(4b) magnetic moment: it is in clear agreement with the neutron data [Phys. Rev. B 98, 134413, 2018]. The calculated contribution of Yb(4b) to heat capacity definitely explains the origin of the Schottky anomaly in the C(T) dependence. A scenario for phase transitions in h-YbMnO3 is proposed, in which the energy gain in the ytterbium system plays a key role. A phase transition at 3.5 K is interpreted as spin-reorientation.