  Recently, alternatives to the Ba\~nados, Silk and West (BSW) effect were
proposed which are (i) due to the existence of naked singularities instead of
the horizon, (ii) require neither horizon nor naked singularity. We reveal the
main features of such alternatives in a model-independent way. The metric
should be close to that of the extremal black hole but the horizon should not
form. Then, one can gain unbound the energy E_{c.m.} in the centre of mass
frame due to head-on collision of particles near the would-be horizon. The
energy measured at infinity can also be unbound. If instead of particles
self-gravitating shells collide, the underlying reason leading to unbound
E_{c.m.} is the same.
