  We show that recent observation made in Grib and Pavlov, Europhys. Lett. 101
(2013) 20004 for the Kerr black hole is valid in the general case of rotating
axially symmetric metric. Namely, collision of two particles in the ergosphere
leads to indefinite growth of the energy in the centre of mass frame, provided
the angular momentum of one of two particles is negative and increases without
limit for a fixed energy at infinity. General approach enabled us to elucidate,
why the role of the ergosphere in this process is crucial.
