  According to the Banados-SIlk-West (BSW) effect, two particles moving towards
a black hole, can collide near the horizon with an unbounded energy in the
center of mass frame. This requires one of particles to have fine-tuned
parameters in such a way that the time component of generalized momentum is
zero $X=0$. Thus the existence of such trjectories is a necessary condition for
the BSW effect. However, it is insufficient since the forward-in-time condition
requires $X>0$ outside the horizon. We examine this condition for different
types of partricles and horizons and find configurations for which the BSW
effect is possible. In doing so, we take into account a finite force of
unspesified nature exerted on particles. It includes relationships between
numbers characterizing the rate with which four-velocity, acceleration and
metric functions change near the horizon. For some aforementioned relations,
parameters of a system control the sign of $X$, in other cases they are
required for $X$ to be real quantity. In the simplest case of free particles
the BSW effect for the Kerr or Kerr-Newman black hole is impossible if a
fine-tuned particle has a negative energy, so in this sense combination of the
Penrose process and the BSW effect is forbidden.
