  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 (CO$_{core}$), and a binary neutron star (NS) companion. As the
CO$_{core}$ undergoes a supernova explosion with the creation of a new neutron
star ($\nu$NS), hypercritical accretion occurs onto the companion binary
neutron star until it exceeds the critical mass for gravitational collapse. The
formation of a black hole (BH) captures $10^{57}$ baryons by enclosing them
within its horizon, and thus a cavity of approximately $10^{11}$ 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 ($e^{+}e^{-}\gamma$) 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 $e^{+}e^{-}\gamma$ 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 $t_{rf}$, 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 Ruffini et al. (2019) 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
(Ruffini & Moradi 2019).
