Within the binary-driven hypernova I (BdHN I) scenario, the gamma-ray burst GRB190114C originates in a binary system composed of a massive carbon-oxygen core (COcore), and a binary neutron star (NS) companion. As the COcore undergoes a supernova explosion with the creation of a new neutron star ( NS), hypercritical accretion occurs on the companion binary neutron star until it exceeds the critical mass for gravitational collapse. The formation of a black hole (BH) captures 1057 baryons by enclosing them within its horizon, and thus a cavity of approximately 1011 cm is formed around it with initial density 10−7 g cm−3. A further depletion of baryons in the cavity originates from the expansion of the electron-positron-photon (e+e−γ) plasma formed at the collapse, reaching a density of 10−14 g cm−3 by the end of the interaction. It is demonstrated here using an analytical model complemented by a hydrodynamical numerical simulation that part of the e+e−γ plasma is reflected off the walls of the cavity. The consequent outflow and its observed properties are shown to coincide with the featureless emission occurring in a time interval of duration trf, measured in the rest frame of the source, between 11 and 20 s of the GBM observation. Moreover, similar features of the GRB light curve were previously observed in GRB 090926A and GRB 130427A, all belonging to the BdHN I class. This interpretation supports the general conceptual framework presented in R. Ruffini et al. and guarantees that a low baryon density is reached in the cavity, a necessary condition for the operation of the "inner engine" of the GRB presented in an accompanying article.