  We use the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) model to
simulate Pb+Pb collisions. In the freeze out geometry of non-central Pb+Pb
collisions we observe a tilt of the particle emission zone in the collision
plane away from the beam axis. We find that the magnitude of this tilt depends
on the scale at which the distribution is measured. We quantify this "twisting"
behavior with a parameterization and propose to measure it experimentally via
azimuthally sensitive Hanbury-Brown Twiss correlations. Additionally we show
that the twist is related to the emission of particles from different times
during the evolution of the source. A systematic comparison between the
theoretically observed twist in the freeze out position distribution and a mock
experimental analysis of the model calculations via HBT correlations is shown.
