  We study the behaviour of an initially spherical bunch of accelerated
particles emitted along trajectories parallel to the symmetry axis of a
rotating black hole. We find that, under suitable conditions, curvature and
inertial strains compete to model the shape of axial outflows of matter
contributing to generate jet-like structures. This is of course a purely
kinematical effect which does not account by itself for physical processes
underlying the formation of jets. In our analysis a crucial role is played by a
property of the electric and magnetic part of the Weyl tensor to be
Lorentz-invariant boosting along the axis of symmetry in Kerr spacetime.
