  We discuss the torque effect in the initial fireball formed in relativistic
heavy-ion collisions, manifesting itself, on the event-by-event basis, in a
relative angle between the principal axes of the transverse momentum
distributions in the forward and backward rapidity regions. The torque follows
from two natural features: 1) the sources of particles (e.g. wounded nucleons)
emit predominantly in their forward hemispheres, and 2) there exist
fluctuations in the transverse distribution of sources from the two colliding
nuclei. On the average, the standard event-by-event deviation of the relative
torque angle is about 20 degrees for the central and 10 degrees for the
mid-peripheral collisions. The hydrodynamic expansion of a torqued fireball
leads to a torqued collective flow of the fluid, which, in turn, yields torqued
principal axes of the transverse-momentumdistributions at different rapidities.
We discuss experimental measures based on cumulants involving particles in
different rapidity regions, which allow for a quantitative extraction of the
effect from the experimental data. We estimate the non-flow contributions from
resonance decays with the help of THERMINATOR.
