  We give simple and general explanation to the effect of unbound acceleration
of particles by black holes. It is related to the fact that the scalar product
of a timelike vector of the four-velocity of an ingoing particle and the
lightlike horizon generator tends to zero in some special cases, so the
condition of "motion forward in time" is marginally satisfied. In this sense,
an ingoing particle with special relation between parameters imitates the
property of infinite redshift typical of any outgoing particle near the future
horizon of a black hole. We check this assertion using the Reissner-Nordstrom
and rotating axially-symmetric metrics as examples.
